Google doesn't miraculously get rights on all books on the planet, they want to secure rights for out of print books and sell digitized copies - sending back their cuts to authors or copyright holders.
But, since they do that on a huge scale (instead of buying rights for one book at a time), this kind of deal requires careful examining - because they could effectively get a monopoly on all out of print books, harming consumers and copyright holders alike.
The hearing is for them to defend how this won't happen with the deals they want to make.
@webbwbb: yeah, right. I recommend you a book: 'economics for Dummies', chapter 2, 'why protectionism is suicide' (right after chaper 1, 'economics and commerce is the exchange of goods').