If you were an advertiser, who would you rather place your ads with?
On the one hand, you have a company that will attempt to gear ads to things like the search history of its users. On the other hand, you have a company that knows where its users went to college, where they work, who they are friends with, what they're reading and sharing, and their favorite bands, books, foods, and colors.
Advertisers want to target their ads to the people most likely to be receptive to them, and information is the key to targeting. The more information available, the better the targeting.