I work at a public library and like any good public library we regularly evaluate our collection, adding new stuff and removing old stuff. We also get a lot of donations that aren't really fit for the collection. Our local friends of the library group gets first crack at all weeded and donated items for their on-site bookstore that raises money for the friends (and thus the library, indirectly), but that still leaves us with a lot of stuff.

Better World Books sends us a bunch of boxes and we can pack them and ship them whenever we need. They generate the label and all we have to do is tape it to the box and send it out with the UPS truck when it comes. No books in the garbage for people to get upset about and presumably Better World Books can do something useful with at least some of them.