Twitter was twitter before they had all those people.
Paying the bills.., IDK. Presumably they've determined that these people are useful in this regard. Otherwise, why have them? But from a first principles reasoning, it seems possible to sell add without thousands of ad sellers.
As an aside it's odd to think of twitter and google etc as a kind of modern successor to the MadMen ad house. I guess if you were being pedantic, they're more like the modern version of what the network TV commercial industry was in the 60's.
Providing this dedicated connection is also an opportunity to upsell them on more ad products.
I did this work at Google for a couple years at the start of my career. They are an exceptionally analytics driven company, and would not employ these kinds of people if there wasn't good ROI