Bingo. It is not exactly better targeting ads. You can sell the same ad (from the same supplier) to the same customer (the person viewing the ad) and the supplier will pay more if they are convinced the ad actually has a higher return rate.

What is the quote again from John Wanamaker "Half the money I spend on advertising is wasted; the trouble is, I don't know which half."

Thus knowing more about their users and what they buy and the ad is successful 1 day, 1 month, 1 year in the future allows Facebook to get paid more.