I have a few reasons for doing this. First of all, I don't want to feel like I'm the "privileged" teacher and I get the best of everything. I make sacrifices for my students, and I think that it's good that they notice this sometimes. If I model unselfishness, hopefully they will pick up on that and begin being more unselfish themselves.
Secondly, I remind them often to take care of their books. If I'm trusting them with a newer, nicer book, then hopefully they will be more motivated to take care of it since I entrusted it to them.
Finally, it's showing respect. The little things that we teachers do to let our students know that we value and care about them do make a difference, although the results may not be something that we see for years--if ever. But, it's just the right thing to do.