|It provides partial protection from all sources of acid and corrosion.|
Preservation is an ego found on cloaks which grants resistance to acid and its corrosive effects. Although unimpressive early on, this ego is useful for fighting the occasional monster with acidic attacks like deep elf sorcerers or yellow draconians, and is especially helpful in late-game areas where acidic attacks are commonplace like the Slime Pits.
A cloak of preservation is good for rounding out your list of resistances, but a ring of resist corrosion or a potion of resistance will do the trick just as well. Many characters may wish to use a cloak with a more broadly useful ego like willpower, if they've found one.