Acid attacks are very rare, but can come from a variety of sources: wading through pools of acid (in general, a bad idea), elemental spells, impact from a vial of acid, or some vile acid spitting creature that has taken offense to your existence.
Apart from instant damage, acid can also destroy valuable equipment. metal, wood and leather can corrode when exposed to potent enough acid. Consider storing vulnerable equipment in rugged containers, so that they are protected from harm.
Protection from acid reduces damage received from exposure to such attacks, but it will not protect your equipment.