What are the advantages to fighting with a 1-handed weapon and a free hand?

Question

So, in a melee combat situation in D&D 5E, I am to understand that there are 4 options as to the variants of weapon-wielding a person can use.

1. Two-handed. A character wields a single, usually heavy/two-handed/versatile weapon with two hands. The advantage of this is increased damage per hit.
2. Dual-wielding. By default (ie. no feats), a character can wield a light weapon in one hand, whilst wielding another light weapon in their offhand. They can use their bonus action to attack using the aforementioned offhand, without applying their ability modifier to the damage.
3. One-handed with a shield. Easy to understand - foregoing damage for a shield to give a +2 bonus to AC. Moreso if the shield is magical, etc etc.
4. One-handed. Here is where my question lies.

Now, as far as I can see, every option has a strength that makes it more attractive than the others for various reasons. All except #4: one-handed. Two-handed allows more damage, dual-wielding allows more damage/versatility, shield allows extra AC, but what does plain old one-handed give?

What I have found so far: classes like ranger can give a +2 damage bonus if you pick the dueling fighting style. You also have the freedom of a free hand during the fight (for grappling, though a two-handed fighter can reasonably take one hand off their weapon to grapple), as well as the ability to use your bonus action for something other than attacking with your offhand. Still, this feels weak.

So, what is there that one-handed fighting offers that other styles do not?

Answer 1

A Free Hand.

In order to Grapple a target, you need a free hand.

In order to cast a spell with Somatic and/or Material components, you need a free hand. (Note: War Caster alleviates the Somatic limit.)

If you want to be able to manipulate objects in the environment around you (without having to sheathe or drop a weapon), you need a free hand.

So, in general, two sorts of character benefit from this loadout.

Gishes

^Slang term for a Spellblade, Swordmage, etc. Someone who fights with melee weapons and magic. If both hands are occupied you cannot cast spells with Material Components. If you do not have the War Caster feat, you also cannot cast spells with Somatic components. This severely limits the spells that you can use.

Thus, a character like a Bladesinger or Arcane Trickster will get a lot of mileage out of 1H only.

Grapplers

You can't grapple if you don't have a free hand, but you only need one free hand in order to grapple. So any character built as a grappler will be best served to use this pairing. You have a free hand to grab your opponent, and then a hand with a weapon in it to pummel them with once you've grabbed them.

Anyone who can't use shields or 2-handers, and wants to save their Bonus Action for something else.

Your statement:

By default (ie. no feats), a character can wield a heavy/regular/light weapon in one hand, whilst wielding a light weapon in their offhand.

is incorrect. PHB 195 says

When you take the Attack action and attack with a light melee weapon that you're holding in one hand, you can use a bonus action to attack with a different light melee weapon that you're holding in the other hand.

Emphasis Mine. By default, dual-wielders must dual-wield light weapons only...both hands. You must have the Dual Wielder feat in order to use non-light weapons in either hand. Thus, a feat-less character with 1H only can use a higher damage main hand weapon than a feat-less character who is dual-wielding. A Rogue (or other character who cannot use Shields or Two-Handers effectively) who wants a higher damage main attack, and to save their Bonus Action for something else would also be well suited to the 1H only set up.

Summary

Ultimately, yes. If you are a pure melee fighter, not using one of your hands to fight is generally a mechanical disadvantage. Most characters whose primary combat method is melee brawling are best suited to make use of both of their hands when fighting. Whether that's 2 weapons, 1H and Shield, or a 2-hander.

But, if you're a gish grappler, or you want to save that Bonus Action (and can't use two-handers or shields)...1H and free hand is powerful.

Answer 2

The advantage is just that you get a free hand. A free hand is useful for many things; it's very important if you want to cast spells in combat, swing from ropes, climb rigging, pick up and carry other objects while you're fighting, grapple, etc. If you don't ever do any of those things, a free hand isn't very useful, and you're possibly better off choosing a different fighting style.

But it's also important to consider that you may have a character build such that it simply doesn't offer you an advantage to do something else. If you're using a certain one-handed weapon for its special properties - for instance, a finesse weapon, so you can use your dexterity bonus - you may not have the shield proficiency required to back that up with a shield, and it may be pointless to you to wield an extra off-hand weapon because you always expect to use your bonus action to do other things, thus just keeping it free to retain versatility is the best use.

As a note, the duelling fighting style does not limit you to a solo one-handed weapon - it also applies when fighting in the traditional sword & board manner.

Answer 3

Not much

Unless you are a 11th level Monk, a Bladesinger, or a dedicated Grappler, you are significantly behind the other options:

• One-handed with a shield. Good defense, and a prerequisite for Shield Master for great control
• Two-handed. You can do better damage, and it is a prerequisite for Great Weapon Master for even more damage
• Dual-wielding. Great damage before Extra Atttack, still better damage than an empty hand after

Exceptions

Bladesingers can't use Bladesong with shields and two-handed weapons. With Bladesong they get much less out of Warcaster, so in my opinion ease of casting trumps 1d6 damage with a bonus action.
Monks get significantly better damage with a quarterstaff in two hands than with Martial Arts until level 11. After that everything has the same damage for them, so they have no reason to keep all their hands occupied.
Grapplers can be really powerful, but only agains the right enemy, and in the right party. Dragging an Stone Giant is nearly impossible, and dragging anyone is mostly pointless if there are no damaging areas.

Special mentions

Rogues are not proficient with shields, and Sneak Attack is not usable with two-handed melee weapons. Still, they lose the most by not using two weapons; if they miss with their main hand, the bonus action attack with the second weapon can still deliver the Sneak Attack damage.

They also can do a lot of things with their bonus action beside attacking. You can argue whether Disengaging is as important as another attempt to deal Sneak Attack damage if you missed first. (I think not, the monster would just attack another party member).
However, it is unarguable that if you have a secondary weapon you can decide on a turn by turn basis. By keeping your hand empty you remove this option, while not getting better at Hiding, Disengaging or Dashing.

TL;DR: Keeping one hand empty makes you weaker in most cases.

Answer 4

A free hand can be used for several things:

• Holding a torch (your eyes in the dark, shield against many dark creatures ánd very effective weapon at certain times).
• Using magic
• Grapping or pushing enemies/objects while in combat.
• Loot (fallen) enemies.
• Keeping better balance, an arm functions like a tail for many animals.

Depending on your rules and master you could always:

• Swing your one-handed sword with two hands for a bit of extra power.
• Loot/pick up a shield for that bit of extra defense power.

There are probably many more options if you think creatively, this depends also on the DM, he can give players out of the box options (they came up with themselves) using a relatively low chance of success (high roll on D20 or multiple D4 dice rolls with same outcome to build up a streak).

Answer 5

Without exploring into later options or strategic benefits, a straight d8 + dueling fighting style does more average damage than a d10/versatile with GWF style. The exact number on that is 6.5 (rapier+2 dmg) average vs. 6.3 (longsword/battle axe/glaive/halberd/etc.).

Now, if you go to the d12 weaps, yes, they pull ahead 7.2 to 6.5 - around 10% (and of course if you're GWF and looking to min-max, you're foolish to do anything but greatsword). But these weapons actually lost ground after style application because they were ahead 6.5 to 4.5 (over 40%!) before such modifier.

So, dueling is there to offset a large portion of this difference. If you want to know "why"? Well, all styles can't be even (unless everything did the same damage) - and some people do still like to play the game and design characters based around concept rather than mechanics. (And if you must have mechanical reasons, they have been outlined above).

For me, I'm always trying to get a 2-handed finesse weapon. The irony is, a d10 finesse with GWF does less damage (without later application of more "stuff"), than a d8 finesse with dueling. Yet, people fight it like crazy. Long live the Elven Curveblade.

Answer 6

Depending on the situation, holding a torch is pretty useful in a free hand.

Also, some combatants consider fighting with one hand a matter of personal pride, and wouldn’t be caught dead (figuratively and/or literally) using a second weapon or even more dreadful, a shield.

Answer 7

It is good if you have a bow, when someone gets close you can draw your sword, drop the bow and still get the dueling bonus, sort of like a surprise, they think you're and archer, yet you are actually a master swordsman and they just walked into your trap. It works defensively too since using a bow will keep you safe most of the fight. Of course, if you are in close quarters or no escape, shield would be the smarter choice.