It’s designed to be your own library. So, you can go through and scan all the books in your house for example and now you can work with them in LibraryThing instead, allowing you to think more broadly in categories and things.
You can also find other people who have the same books in their collections and see what else they have in theirs, to give inspiration as to directions you might want to go in.
You can sign up, scan the books you own (the bar codes scan nicely – I got every book in my house scanned in a day or two), and there’s something nice about seeing it all there in one place.
I’m using it in an odd way — its support for Dewey Decimal suits me perfectly so I’m using as a mostly hypothetical textbook library. I just started about six days ago because they opened it up now. [used to be 200 book limit before membership: now it’s free and I’m already past 480 books).