Think about how you use a book. You want to read it, sure--but there are a host of other ways for you to interact with the words between the covers. You might want to flip through the pages to find an image. You might want to open right up to the table of contents so you can find your favorite chapter. And you might want to pass it along to a friend so they can have a look at it, too.
Google books attemps to offer several interactive and innovative features to reader.