Dipping was a feature in DOS and as BG3 is just a modified DOS2 they just did not remove this mechanic, probably because Larian still thinks its "cool".
So it's an
inherited stupid mechanic?
Yes.
Larian likes their systems, and their philosophy seems to be to not punish players for making stupid decisions. Dipping your weapon in certain substances make sense, but rather than keeping it logical, they seemed to prefer to apply some beneficial effect no matter if it makes sense or not.
