I assume the logic is that it eats away at whatever is protecting you making it more likely for an attack to hit an opening (Or to pierce through softened material)
Beyond that, it's more "Game logic" than "Actually makes sense logic" in that it is applied equally across all sources of AC (Since BG3 doesn't bother to track item materials, which is why Druids can still cast their spells while wearing full Plate armour when normally their spell usage is restricted when wearing non-wooden/leather equipment)
The current iteration seems to be more catered towards being less punishing than traditional "Acid" in DnD (Which is not always the chemical compound known as acid) that is notable for creatures such as Green Slimes and Black Puddings, wherein it would actually degrade armour and weapons and ultimately destroy them.
Thus it ends up being more player-friendly while still maintaining that theme of equipment degradation (Albeit in a entirely useless way thanks to Larian and their surface mechanics where acid pools last for all of a single attack upon which a creature bleeds and replaces the acid surface with blood...)
