Preservation is an ego which grants you both the conservation and resist corrosion intrinsics, as well as one rank of acid resistance. Although unimpressive early on, this ego is extremely useful in much of the late game where fire, cold, and acidic attacks are commonplace. These areas also have many sources of mutation, and as the most common defense against these is the amulet of resist mutation, having a cloak-slot item that duplicates two other useful amulets is extremely helpful. Even so, consider replacing it with something else when attempting branches which pose little risk to your items, such as the Crypt or the Tomb.
Preservation has no effect on the food-stealing attacks of harpies.
The preservation ego can be found on the following: