Usually the main job (with main job I mean core purpose of the class) of a class is quite clear to me. A fighter damage, a wizard magic, cleric healing. This is superficial I know (and there are heaps of archetypes but to me every class still has its main purpose. But the purpose of the magus is not really clear to me. In what case is a magus necessary within a party and what would be his job?
The magus is, ultimately, a fancy fighter. But the differences matter.
For the basics, the magus gains four major class features:
Spells. One way of thinking about the magus is that the magus uses spells like the fighter uses feats, accepting per-day restrictions in exchange for greater power and much greater variety. This is true, but spells also mean much more than that; see below for more details.
Spell Combat. This is what allows the magus to do what it does; while the spells are the workhorse of the class, spell combat is the iconic feature that makes a magus a magus. Basically, magi get to two-“weapon” fight, but with the offhand weapon instead being a spell. This means the magus can fight and spellcast simultaneously, which is huge.
Spellstrike. The magus can turn a touch-attack spell into a weapon-attack spell, combining a attack with the spell effect. This is nice, but not actually all that critical; one additional application of your weapon damage doesn’t matter so much, and a magus should feel comfortable not using this if a non-touch-attack spell is better in a given situation. At early levels, though, using spellstrike on arcane mark (an at-will touch-attack cantrip) can turn spell combat into actual two-weapon fighting, which is sometimes all you really want.
Arcana. These are various side-benefits that make your magus your magus, different from others. Some of them are quite good, though like most of the “ability off of a list” features, they’re very hit-or-miss.
About fighting and about spellcasting
So on some level, the magus is this fighter who is using magic instead of bonus feats to be better at fighting. But magic is very nearly equal to power in Pathfinder. Being a fighter who uses magic to be better at fighting, rather than a fighter who uses skill and training to be better at fighting, makes you a better fighter, and it makes you a much better adventurer.
Even with a relatively slow spell progression, magi get many, many more spells than a fighter gains feats. That means a magus is much more capable of having answers to a variety of situations—both combat and non-combat. These spells can make the magus fight better than a fighter can (by having more options available), and make the magus far more useful outside of combat.
So the magus’s usual role is that of the fighter, but a fighter who is more versatile, more capable of dealing with the variety of challenges that an adventurer faces, and less forced into a singular niche. Against extremely vanilla opponents (who pose no special challenge to a fighter), the fighter is probably more effective, but especially as more levels are gained, such enemies become rarer and less threatening overall; most real threats do have some special challenge that causes problems for a fighter. Because a fighter can only prepare for so many things, while a magus can prepare for many more things than a fighter can, the magus is more reliably useful, more useful against greater threats, than the fighter is. And outside combat, there is no competition.
A downside: magi multiclass poorly, but then so do most Pathfinder classes
There are some downsides to the magical approach to combat, though: it does kind of lock you into the magus class. You qualify for feats (mostly) independently of class, so if you take a level of fighter at 10th, you can get a 10th-level feat. But you only qualify for better spells (and, though it’s less critical, better arcana) by taking more levels of magus. So if you take a level of magus at 10th level, instead of getting a 4th-level spell as a 10th-level magus would, you get a 1st-level spell. Therefore, magi are not as good at multiclassing. Ultimately, however, Pathfinder punishes multiclassing massively, and the only time it’s really viable is when you can get something particularly potent from a level, maybe two, of a class, delaying some other primary class only a little. The magus class features only work with magus spells, so it is not good for this purpose.
About “role” and how there’s not really any such thing in Pathfinder
Finally, a last point about “role”—Pathfinder doesn’t really have any such concept. Above I’ve described the things that a magus is natively good at, but that isn’t necessarily the same thing. I’ve tried to emphasize, even, that much of what makes the magus better than the fighter is his ability to contribute to non-fighting things. But the fighter’s “role” cannot accurately be described as “damage”—the fighter can be built to do one or two of many things, and a given fighter may not be interested in damage at all, instead focusing on defense or combat maneuvers or intimidation or what have you.
Meanwhile, both cleric and wizard can be built to do far more damage than the fighter can. Clerics, in particular, can be so much more than just healing, being one of the most powerful and well-rounded classes in the game, capable of dealing damage in melee or at range, taking hits, buffing allies, curing debuffs and damage, and even summoning assistance. And by the same token, while “magic” can cover what a wizard does, that term is misleading: “magic” can and does cover just about everything.
So I also recommend that you avoid pigeonholing classes in this manner. Classes have strengths and weaknesses (well, most of them; some of them really seem to lack any real strengths, and others real weaknesses), but they mostly can and should take on many roles—and which roles are as much up to the character, the campaign, and the individual choices made in character creation, than it does to do with the class used.
The magus blends both the mage’s arts and the warrior’s arms with devastating results, slicing apart foes and blasting them with eldritch f lames[...] (Ultimate Magic, pg.8)
Role: Magi spend much of their time traveling the world, learning whatever martial or arcane secrets they can find. They might spend months learning a new swordfighting style from a master warrior, while simultaneously moonlighting in the local library, poring through tomes of ancient lore. Most who take this path dabble in all sorts of lore, picking up anything that might aid them in their search for perfection. (Ultimate Magic, pg.9; PFSRD - Magus)
This doesn't really say much about what a Magus should be doing, does it? But what does it really tell us?
Magi are adept adventurers, constantly seeking new skills and knowledge, creating unique ways to combine and apply all that they learn. This makes them a jack of all trades, but master of none. They are a versatile bunch, capable of filling many rolls as required by the party. Most would focus on their blend of the martial and the arcane: They can be heavy hitters when played this way. But the real strength of the magus isn't in Spellstrike nor Spell Combat, it's in the Arcana.
The Arcana is the source of the Magus' versatility. The secrets they have uncovered through their travels. With these pieces of knowledge, we can create a character for several different roles. Including, but by no means limited to:
Magically Enhanced Fighter
Many of the Arcana focus on improving the natural abilities of the Magus. Improved speed, accuracy, and protection are all available to the Magus through the use of the Arcana. This allows the Magus to switch tactics on the fly, to outwit and outmaneuver any enemy. The Magus has many tricks up their sleeve in order to keep enemies guessing. Aside from the obvious Arcane Accuracy, Hasted Assault, or Maneuver Mastery, clever use of Prescient Attack, Divinatory Strike, Bane Blade, and Spell Trickery can give you the edge you need to tip the balance of any encounter in your favour.
Many Magi, in the course of their travels, learn how to counter other spellcasters. They focus on overcoming and dispelling the magic of others, while keeping their own faculties in check.
Through the use of Arcana such as Arcane Scent, Concentrate, Dispelling Strike, Disruptive, Lingering Pain, Reflection, and Spellbreaker the Magus becomes an effective hunter, and slayer, of Wizards. This can be an invaluable addition to any party set to face such dangers.
Sometimes a party may need a deft eye in place of a strong arm. Some Magi learn to enhance their senses, and how to use their magic to hide and disguise themselves. These Magi know the secrets of the Arcane Cloak, Dark Shifter, Distant Spellstrike, and Ranger Trap. Many also have Familars to offer yet another set of senses.
The Arcana often present paths of knowledge even deeper into other domains. Skills gleaned can become skills mastered, with enough time and effort. There are Magi who strike with the force of a Monk, using Ki Arcana. Some have spoken of shapeshifted Druids delivering spells along with blows from their deadly claws with Natural Spell Combat. Magi who fight as gallantly and flamboyantly as any Swashbuckling pirate with their Arcane Deeds. There are also those that have decided to delve deeper into the arcane: Broad Studies, if you will.
But what is a Magus?
That Magus, in the end, is whoever they want to be. No Magus is the same as another, and it is their goal (and their pride) to find and forge their own destiny. They are an ever expanding bag of tricks learning new abilities, spells, and tactics as they adventure onward.
Well, I just played a scenario where the two sorcerer 1/magus 2 characters (nearly identical) combined to do the following: One "blocked the hole" while the other used ranged spells (full round action reach metamagic with a 4d6+8 shocking grasp). Now, granted, we're firing into melee because the first is blocking, but the blocker is also doing SG (in melee, again 4d6+8). So at 3rd level, even though our "to hits" blow chunks (-1 total for the range attacker, against touch AC, +1 to -1 for melee depending on how they are casting) they are against touch and do bring things down.
Now, the added benefit of a 3rd Magus spell was "Vanish" for 2 rounds. Now the melee or the ranged can vanish, cast full round metamagic spells from sorcerer, or say true strike then attack, then vanish to buy the team some time (this is what happened: with the 2 rounds of vanish being used to tap the tankbear a few times to get him back awake). Dealing with 3 ghouls while blind at 3rd level can be difficult. Luckily I rolled 72% on my concealment miss chance, so the true strike / reach shocking grasp was not wasted. Vanish changed the TPK (most likely outcome if I'd missed) to a win. But only because my partner magus was blocking. In the future, once we get a little more experience we'll be even worse (ranged attacks against touch ac while invisible = +2, ignore DEX (the only thing you really get to touch AC). So we're like +5 THAC0 vs a 10 on most days, take 4d6+8 or 5d6+10 (until we're up a bit then it will double). Of course we still have the option of a +1 weapon (any we pick up), as well as delivering spells (SG) through said weapon. We're running around with Cutlass' atm (18-20 crit with SG spells, sure, please 8or10d6+16or20).
In reality, except for the whole "uber zapper" I wouldn't find the class very appealing. The BAB hit is just to much for the damage output. Being able to go against touch AC (and use reach spell - thanks magic lineage) is highly appealing.
"Zapping fire support" is what I would categorize our characters as atm.
I prefer the spell dancer magus to all others, but to me, the magus's primary job is eliminate enemy spell casters. Their bonus to AC against attacks of opportunity and then having magus arcana to make it more difficult to cast spells (lingering pain, etc) they are the perfect weapon against enemy spell casters. Dimension door on opposing spell casters at high enough level. Keen that weapon before you go and surprise with a critical strike. The Magus are the unknown element to any fight, bypassing meat shields go toe to toe with enemy boss!
Magus are damage dealers. They have 2+Int skills, so they could theoretically be skill characters (with a +4 or +5 Int bonus) to some degree.
If you "uber" out your background you can do the following: Get +3 CL to Shocking Grasp at 1st level. That means a successful strike will hit for 4d6 at first level. This scales as you go up in level.
Add to this a dip in Crossblooded Sorcerer and now you are seeing in the dark (orc/draconic) and adding +8 to the damage (again, it scales). Which means you can make a human sorcerer/magus who can do 4d6+8 with sorcerer SG and Magus SG. And as you go up in Magus, you gain dice. Even so, a second level player attacking against TOUCH AC doesn't need a high str - a +5 Str min/maxed fighter just about equals a touch magus "to hit". And would you rather be doing 2d8+5 (best fighter weapon?) or 4d6+8? And you still get to critical the damage, in fact it is EASIER to crit fish with a Magus.
Why? Two abilities: 1st level spells/wand of True Strike adds +20 (this applies to confirmation rolls as well). And the second is Spellstrike with a KEEN weapon - +2 bonus total, so 8000 gp OR you can wait until 5th level Magus and use ANY weapon with an 18-20 crit range because you can make it a +1 Keen. So now you are dealing 10d6+20 damage on a crit.
Oh, but it gets better... take a trait called Magic Lineage and now you can apply Intensified Spell without even raising it from a 1st level spell. So a crit now does up to 20d6+40. Not bad for a first level spell (this happens around 7th level Magus, give or take). And you can recall it a bunch of times a day (so you get 4 casts from Sorcerer, 4 from Magus, and 5-8 from Magus' spell recall).
Now. If you really want to destroy the dice bags out there, you start training Wizard - straight wizard. And you do this at 7th level (before taking a 6th in Magus). This makes your Magus 8th level Magus Arcana Broad Study with Wizard Spells. Then you level up to 11th Wizard (18th total, pick up spell perfection at 15th level for Shocking Grasp - reach and elemental are good adds as well). At 11th level wizard you take staff-like wand, which lets you use your spell CL on wands instead of the wands. Now you craft 750 gp (350gp to craft) Shocking Grasp wands that all go off at 10d6+20 damage. 7.5 GP a pop... and yes, you can still spell strike with the spell even if it gets cast through a wand (so 20d6+40 is still easily possible... 1st level spell). Yeh...
So in conclusion: Magus are damage dealers with magic abilities. Vanish; true strike; critical spellstrike... all nice low level spells... wand of true strike is 750gp, so you can use 2PP in PFS to get one and not waste a spell slot. That is +22 to hit (+20 for invisible and +2 for flanking if you do it right). Great crit fishers!
Clerics don't really, not always, do any healing. In pathfinder you have to touch a person to heal them. While this is fine in the lower levels, it gets harder as you go. And their domain spells are relatively weak. I trust my clerics to patch me up, however, even though they haven't got that godly healing all healer mages face in other fantasy lore.
Instead, I like to think of clerics as buff machines. It is better that way. They buff you, debuff enemies, remove debufs YOU might have, and they bring your hp up to speed whenever necessary.
Now, figthers have 2-3 archetypes. You have a damage dealer, a tank, a rogue and one archers. Fighters, like clerics, don't necessarily fit in into any of those because they can technically do all of them. Only that they specialize in one in particular. wizards? wizards, arcanists, and rogue tricksters can do everything you want them to. Their only real disability is the low amount of attack bonus they get. Meaning they won't be as good in the crossbow as any fighter, or even druid. They have abysmal hd. Sitting at d6 per level. And are overral just pitiful front line fighters, but not having attack bonus makes them bad at the bow as well. The game would still not limit you, and it won't limit the wizard. The wizard can use plenty of great buffs, like legendary proportions, mind blank and mind blank communal. They have a shit-ton of options for specialization. If you need a figther? go invisible and summon a bunch of tentacle monsters. If you need a cleric? have your wizard study necromancy, bring some potions, a bless and prayer spells in scrolls - good to go. Damage? fireball to your FACE. Wizards though are really limited by the game system. You even have a maximum class level for every spell after which your spells of lower levels won't grow any stronger. Making them obselete. where was I going with this? right, magus. It is very much in the spirit of the game. As it won't limit you, only wizards so they can be balanced around fighters. A MAGUS can hit with a sword. They can cast up to tier 6 level spells. The same as the spellcaster archetype of rogue.
They're essentially wizards who don't go full on magic. Making them spellcasters. If spell damage wasn't limited by level, they would molest fighters. They are much cooler than fighters in that prespective. While not being as adapt with a weapon, they're nearly as good if not more. If magus could combine his spellbook with that of a wizard, I would take a few magus levels myself, up till my caster has nine level spells.
Think of it this way. In real life, and real live characters, people would often choose to learn several things instead of just being a 'fighter'. So every archetypical character can and will fullfill several roles. At least in pathfinder, as you are roleplay this game. This has very much that feel of 'real life mechanics', they will do several jobs. Despite them not knowing to. And a magus is just a bit less of a fighter, comparitively, but he has weapon enchantment, and can free cast spell along with weapon damage. Making him a very effective damage dealer. There are several utility spells like displacement or haste he will pick up. Bears endurance, or owl's wisdom. Touch attacks that deal extra of that sweet damage. Compare it to a fighter who can just 'hit things more effectively' then you have magus lying down and taking the throne.
Wizards are beasy to waste time on some debuffs and utility spells. No can do on their part.