Isn't it obvious?
Presumably, the books are going to be bought but then kept in the cloud (much like Amazon's Kindle offering - the books can be deleted off the device at any point but they're available from the Amazon site for re-downloading.
It wouldn't make sense for Google to pursue a strategy of putting everything in the cloud (see Chromium) but then pursue downloading books onto peoples' hard drives (to do so would be presumably short-sighted). Books are small enough to be cached to their new Chromium devices for off-line reading, so it makes an ideal first step for them
Ultimately, they'll provide your personal bookstore using your google account. Anyone who's buying a book from the Google store for the first time will be required to set up an account with google, meaning they then have more detailed information about you than they previously did. Information is power. The other thing is that they may be providing a subscription based model - how about paying £10 a month to read as many books as you like? Works with Spotify...
Biting the hand that feeds IT
