1. getting a warrant and running down to the book shop to ask them about what a single suspect bought, and
2. Having enormous amounts of data collected about hundreds of millions of people, processed and interpreted, essentially sitting in one place. Whether Google voluntarily gives it up or not is almost irrelevant (as we saw, the NSA is happy to tap Google's private fiber without their knowledge even when Google is cooperating on other fronts).
Of course the government will find ways to be evil if it wants (and I think that we're generally lucky because it doesn't seem to want to with any particular intensity). But that's not really the point here.
It's the difference between having many small, complicated little targets that each yield very little information, versus one conveniently enormous target that yields information on everyone.