I have such catalogues on other services too. But don't really know why or how they are useful

. Well, it's the same with all the social networking sites that I have accounts on anyway.
Unlike the general social networks, the likes of LibraryThing are set around something that you are hopefully passionate about. This makes LinraryThing useful even without the social aspects in it. My LibraryThing catalogue, for example, tells me that I have more unread books than otherwise

. Not to mention the useful social features like reviews/comments from your friends.
LibraryThing also offers some nice
widgets that you can use to show off your collection of books (or what you are currently reading) on your blog or on other social networking sites like Facebook.
PS: LibraryThing was the first of the services, all others evolved from this idea. Not that this matters much, but just wanted to point this out; I am a big fan of LinraryThing the service as well as LibraryThing the company.