It's related to how the internet ad market has evolved. Initially, sites would run their advertising side kind of like newspapers do - have in-house departments

It's related to how the internet ad market has evolved. Initially, sites would run their advertising side kind of like newspapers do - have in-house departments to sell ads directly to the companies that want to advertise, then place the ads on their pages manually and serve them from the same servers. It works, but it's a ton of overhead to have all those ad sales people and find companies one at a time willing to set up a direct relationship with your site and deal with direct billing etc. It was kind of a revolution to set up these advertising middlemen. Site owners just sign up to show ads and set up their site to load an ad from the ad company's server. Advertisers buy their ad time from the middlemen too. Nobody has to maintain those high-overhead direct relationships. Huge boon for the little-guy sites on both sides. That's the good part. There's plenty of bad parts too, including monopolization of the market, abuse, poor behavior of anti-abuse mechanisms, etc. It's what we've got though, until somebody comes up with something better.