I have a bunch of old English literature books sitting in tubs in my basement, waiting for me to find time to list them all for sale online somewhere. In the meantime I have these books, in addition to all my other books/music/videos, cataloged in DL. I took photos of each individual book and added them into DL so I can see at a glance what the book looks like and what condition it's in without having to dig it out of its tub. I also labeled each of the tubs, e.g. "Tub 1," "Tub 2," etc., and added this information into DL so when I need to retrieve a book, I know right away which tub it's in.
Excel could do this too, but DL is just neater, and the integration with Amazon is nice, although it could be a lot better. I would appreciate a way to list books for sale on Amazon/Abebooks/Biblio/whatever and have DL keep track of everything. And I think DL just needs more features in general, like report generating and more advanced searching. But it's still young, so I'm sure better things are coming.