Your library has a public URL you can share with other people and even a feed. You can also write reviews, rate the books or categorize them using tags. Probably the most important reason you should build the library is because it becomes searchable. Imagine being able to find a scene from one of your books without knowing its title and by typing some keywords that describe the scene. Of course, Google didn’t index all the books in the world, so many of your favorite books aren’t yet searchable.
While this function does have an import available, it was able to find 138 of my 650+ titles that I exported from LibraryThing. I think this is handy for the reason stated above – being able to search full text, but I’ll stick to LibraryThing for cataloging my books.