I'm sure the thief stole them out of greed, but didn't know who to sell to. Now the people buying and owning those books keeping them in their private collections, they typically do this out of vanity I would argue similar to people owning expensive art (although some do this also purely for investment).
A great book talking about the secret work of rare books is "the club dumas", which I highly recommend. The movie was quite bad on the other hand.