An annoying ad today might make you try Anacin tomorrow. But Google had something different: access to people who were plainly expressing a commercial intent: when someone types in “drug rehab,” “male pattern baldness,” or “find a mortgage,” there’s little doubt what’s on their mind. The new system would therefore show relevant advertisements only at those moments, and—quite radically—stay out of your way otherwise. In short, Google took the decision to leave a lot of attention on the table; the theory was that advertisers would value surgical strikes at least as much as carpet bombing.

Tim Wu, The Attention Merchants, loc. 889