And you need hundreds of thousands of those every single day. Not enough eyeballs. In short, selling sponsorships. It works the same way in a website. A savvy marketer knows that the web consumer is a different breed of cat.
Sponsorship goes a LONG way in communicating that to consumers, which in turn, cultivates loyalty.
This means the sponsor has to commit to a longer term plan, which means you're generating revenue on a more stable basis. This, in and of itself, is not a bad thing. Positioning-wise, you can't beat sponsorships for class. When a sponsor takes over a whole page, his message is less rushed. When his message is less rushed, it's often better read, which means people give it more time and attention.