I've always wondered about that too. I would have thought that people would leave behind books they didn't find interesting and pick up ones they did as a way to 'upgrade' their personal libraries. But that could cause a problem whereby public bookcases would quickly become filled with uninteresting books that no one wants to read. Leaving aside the fact that people have different taste in books, I think that there are certain power users who fill the bookshelves up with books because they derive a certain satisfaction in promoting literature and sharing books they've enjoyed. I've seen a few of these people , who would come with giant bags and fill the bookcases up to the brim. It's almost like a vocation for them. But without these major contributors, the bookcase would soon lose its dynamism and oxygen, so to speak. I see them as a bit like philanthropists who fund the arts and culture-based websites; were they to disappear, the programmes they support probably would too