What I've seen from my own personal experience, is sites that sell advertising space have been tracking their advertising since they started because they need to know the value of their real estate.

Over time, this transfers to a fact that where ever you are doing your ad banners (a web site, a blog, a search engine), the people you are buying from know how to reach their audience best. Serious advertising firms have their own creative-content guys on the inside, and using them in those cases is way better than picking an external designer, freelancer, or using your own guys. This kind of comes down to this:

"If someone has a big yard, the kids that live there know how to play in it best."