A webmaster wants to sell ad space, but doesn't want to go and court each and every marketer buying ad space who will just rebuff individual proposals anyway.

A webmaster wants to sell ad space, but doesn't want to go and court each and every marketer buying ad space who will just rebuff individual proposals anyway. A marketer wants to buy ad space, but doesn't have time to sift through millions if not billions of websites and court their webmasters one by one. Ad platforms bring together the webmasters and the marketers with a one-stop shop. The webmaster courts Google and gets ads to sell his ad space to. The marketer courts Google and gets ad space to put his ads on. TL;DR: Efficiency and logistics. Capitalism ho!