It's not to do with mechanics. It's to do with the narrative. You describe attacks differently depending on the type of weapon you are wielding.
Throg the barbarian whirled his mighty warhammer around his head and uttered the battle cry of his tribal forefathers but the hideous orc merely curled its upper lip in a sneer as it raised its iron shield high to counter the blow. The shield bore the signs of many such hammer-blows. It has stopped them all.
"I stab it with my hammer," announced Throg, "Upwards under its shield, straight into its ribcage."
"Wut?" said the DM.
"Well, you house-ruled away damage types, so if I can do stabbing with a dagger, I can do stabbing with a warhammer, can't it?
"Er ..."
On the other hand...
"Why can't I slash the monster with my short sword?"
"Why can't I pierce the monster with my longsword?"
Having recently gone back to read an AD&D Monster Manual, it took significant space to note each unique interaction, and even those were sometimes unclear because there was no standard format or phrasing. Here's a quote from the book I was using:I dunno, we did okay in bD&D and AD&D 1e without damage types.
I dunno, we did okay in bD&D and AD&D 1e without damage types.
AD&D 1e certainly had different kinds of damage, they just weren't formalized.
It's a bit of an exaggeration, but in general, it seems monsters are immune to a whole bunch of damage types or none at all.
Most notable are the non-magical types: how many creatures are specifically resistant to one of piercing, slashing, or bludgeoning rather than all three? Very, very few. Weapon choice is much less interesting when any weapon is just as good as another.A
voiding resistances is trivial when there aren't any or they are all clumped, or only fire matters. On top of that, vulnerabilities are also extremely infrequent.
Kinda makes damage types pointless for most encounters.
Why did the developers make damage types so lame?
Sure, but that's was kinda the point—these editions had no codified "damage types". Instead, they merely gave monsters specific vulnerabilities and immunities. (sometime very specific ones, like rakshasa and blessed crossbow bolts), and these ultimately served as the inspiration for, and gave rise to, the (slightly) more modern concept of damage types, but it's rather anachronistic.

ro said:Most notable are the non-magical types: how many creatures are specifically resistant to one of piercing, slashing, or bludgeoning rather than all three? Very, very few. Weapon choice is much less interesting when any weapon is just as good as another.
You're splitting hairs.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.