The thing that made it click for me while watching Wolfram's presentation was emphasis on computable knowledge. Google excels at finding anything that someone, somewhere, has explicity written out. With a few narrow exceptions, it does nothing to help you synthesize information. Alpha is likely to be a poor source of finding most kinds of information but an excellent source for synthesizing what information it has.

In other words: Google can help you find things like a phone book can help you find things, Alpha can help you find things like a calculator can help you find things.