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?
But yeah. dipping (coating) weapons in venom/toxins has a fairly long history. Just about the only time I've used it so far was when I noticed there was a puddle of toxin (or maybe venom? Forget which. Conceptually wise should be venom, but mechanics wise may have been labeled toxin) on the ground I could dip weapons into by the body of a dead phase spider. Now THAT actually makes sense.
Though it should probably last for a set number of hits instead of "until long rest"... And probably shouldn't do much for bludgeoning weapons.
