I used to be editor-in-chief of my college newspaper. Most of our print ad revenue didn't actually come from direct sales. It came from national agencies that gave us insertion orders. I don't think we could have had a print edition based on our in-house ad sales.
Now imagine this problem for all kinds of other niche websites that can't afford to have their own ad sales teams... Suddenly it becomes clear why outsourcing ads generates a lot of value for cheapskate media consumers and for cash-strapped media organizations.