A lot of book releases are limited to a certain amount of copies e.g. Arion Press Moby-Dick that was limited to 250 copies. Adding a field for "number of copies in edition" would be good for book collectors, especially in cases where there are multiple collectible editions released around the same time (example: 10 copies with an autograph and a piece of art, 100 copies with just an autograph, 1000 normal copies).
Sometimes, these editions are numbered so that you know which number you have e.g. the edition is limited to 100 copies, the one you buy has a stamp that says "#43." If a book has this field and is added to your collection, another field could be added next to its entry that lets you mark which number within the run you own.
Relevant thread: https://community.metabrainz.org/t/fine-press/691142?u=naufragiosus