What is the point of weapon type resistances?

Question

I recently read the monster manual and every creature I came upon that had resistance or immunity to bludgeoning/slashing/piercing damage followed this with "from non-magical attacks" (may not be literal since my handbook is not in english)

What this boils down to:
What is the purpose of those resistances, since they are easily overcome by items experienced PCs tend to carry. Magic weapons.

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Everything below provides context, but is not necessary to understand the question

The complete story behind this question is that I want to provide my players with a challenge in a dungeon. 5 monsters, and each can only be deafeated by the specific skills and abilities of one player.
While designing this I noticed this problem.
Since the point is NOT having the rogue sneak-attacking everything to death with his magic weapon, and I also prefer not to solve problems with houseruling I put this question.

Answer 1

Not everyone has a magic weapon

The type of loot and magic items to hand out is largely dependent on the GM. While your campaign might have magical weapons for all PCs, in mine usually only one character who regularly use weapon attacks have magical weapons by level 5. That's plenty of time for the non-magical damage resistances to kick in. I'm sure there are GMs who are even stingier than me in that sense. It's a matter of high versus low magic in the setting of the campaign, and varies between tables and games.

When the party has few magic weapons, or the magic weapons are unreliable or otherwise weaker than their mundane counterparts, the mundane weapon type resistances can create for interesting encounter setups where some PCs are dramatically less effective against some creatures than others. This can cause nice variation for a party that's bored of the more straightforward encounters.

I mean literally not everyone has a magic weapon

Assuming every experienced character has a magical weapon, what about the non-adventurer armies of the lands? While the leaders or champions of such armies could plausibly have magical weapons, they're usually rare enough that not every conscripted peasant would receive even a +1 spear. Taking this into account, the mundane weapon resistances serve a purpose in the fiction - they're a reason why two hundred armed villagers cannot compensate for the adventurers when it comes to hunting creatures with these resistances.

Flavor

The origins of many of the DnD creatures with such resistances carry descriptions of the creature having a supernatural immunity against usual weapons, and conversely being vulnerable to specific ritual weapons only (eg. silver bullets or wooden stakes). Maintaining the damage resistances is a way for DnD to pay homage to the original stories the magical creatures originate from, and provides flavor even if the rule is never expected to kick in.

Answer 2

Your question says that "they are easily overcome by items experienced PCs tend to carry. Magic weapons.", but this makes an assumption that magic weapons are common. If they are in your world and game, then indeed the resistances become almost meaningless. However, if we take a look at the DMG on starting at higher level (page 38), you can see something about the expectations the game makes about how many magic items a PC might obtain during his adventures.

And you will see that it's not as easy as you're thinking.

In what the DMG labels a "low magic" or "standard" campaign, characters of level 10 or below start with no magic items at all. Only in a "high magic" campaign is a character between the levels of 5 and 10 allowed to start the game with one magic item. And it might easily be something other than a weapon, of course.

Unless your characters are very high level, or your campaign is very high magic, the idea that every character owns a magic weapon is out of line with the expectations the game makes.

As such, under the game's normal assumptions, these resistances stay relevant for quite a long time. They will require PCs to find ways to defeat these monsters, or prepare spells for them, or whatever.

(At very high levels, it won't matter to the PCs that the monster is magic resistant, but it will provide great flavor and make it very hard for the rest of the world to deal with it. A few hundreds arrows from a trained army can down even the most powerful monsters, if they don't have protection from non-magical weaponry.)

Answer 3

Magic weapons are not as common as you seem to think.

Depending on whether or not you're running a high-, mid-, or low-magic campaign, your PCs might not have access to magical weaponry until the 3rd, 6th, or even 11th level! This allows monsters to get much more use out of their resistances early on. The reason those resistances exist are two-fold:

1. 5E encounter difficulty relies more on HP damage.: The way 5E works when determining the strength of a monster is by looking at its HP more than its AC, due to bounded accuracy. In case you're not familiar with the term, bounded accuracy means everyone has a chance to hit, regardless of the difference between levels. A level 1 PC could technically hit the Tarrasque. However, since such a monster is immune to non-magical physical damage, no matter how many times it's hit by a low-level PC, it won't go down.

2. This allows the PCs to feel more heroic. As far as who has access to magical items, well just consider that a +1 weapon should cost between 500 anf 5000 gold. So while the noble hero who rescued the mayor's child might receive an ancient heirloom that does some magical damage, 99% of the population won't have such luck. Think how much it'd cost to outfit every guard in Baldur's Gate (some 10,000 guards), with +1 swords and maces! You won't come across too many of these kinds of weapons in the world, and wielding one is meant to give a heroic feel to a character by saying: "Haha! Your puny defenses, while effective against most, will not slow me down, monster!"

As a side note. I am just talking about plain old resistance or immunity to non-magical physical damage. For the resistance or immunities to non-magical damage save for a certain material or weapon type, these are present to either reflect the lore from where the creature originates, such as with Lycanthropes and silvered weapons, or the natural defenses of a monster, such as with Treants and their resistance to piercing and bludgeoning.

How can you circumvent this? Well, you have a number of ways. My personal favorite is throwing in a shambling mound to eat up the rogue, or a black pudding that can also affect magical weapons. But there are other ways as well, such as throwing in certain terrain to prevent flanking or allowing only one of your PCs in melee range at once, giving your rogue disadvantage on his attack any number of ways to prevent sneak... You have plenty of solutions available to you should you want them.

Good luck!

Answer 4

In addition to the "not everyone has magic weapons" answers, in the greater context of the story, the immunity to non-magical weapons also offers a shortcut explanation as to why that monster is still around and has not long since been dispatched by the locals, even if they need to attack with 20 people instead of 5 PCs.

Secondly many characters also carry multiple weapons, and an immunity to non-magical weapons restricts their weapon choices to their (typically) main armory, reducing their strategic options and sometimes forcing them into thinking up better strategies instead of picking up that other weapon. Combined with disarm or other actions removing their (at this point only) weapon, it is easier to put the PCs in a tough situation.

Answer 5

Let's approach this from a game design perspective, which is usually inspired by the real world.

Hitpoints is a exceedingly simplistic model of life, death, and harm. As a human, I die if I get cancer or a heart attack, or get hit by a car, but those are all quite different and more interesting than just going from 37 hitpoints to -10hp. So to make a game more flavorful, it's be nice to have at least a little more variety.

We don't have dragons, demons, slimes, in real life but we do have monitor lizards, tortoises, condors, and jellyfish. If you take a rapier to a tortoise, you get a very different effect from using it on a jellyfish. You can probably club a jellyfish, but who knows what would happen if you club a tortoise. So damage resistance or immunity may initially have been inspired by real animals.

Varying the animal resistances is probably also intended to give a point to the weapon list of 50+ weapons.

So what can you do?

If the benefit of having additional variety & complexity in your monsters sound compelling to you, you might want to carry over those immunities as resistances when the same damagetype of magic weapon is in play, to keep enemies more diverse and different weapons more useful.