If you are a large traffic source, more likely than not you are using a combination of top tier advertiser feeds, such as Google, and second tier API or XML feeds to monetize your traffic. These solutions are appealing because they are essentially automated solutions, and require little or no interaction with the end user advertiser. You do not have to know who is using the traffic for what, and issues such as billing and payment are all done with known contacts.
The problem with these solutions is, of course, that they offer smaller margins and less control and interaction with the end user. And as a result, the advertisers can pull campaigns and stop advertising because of small margins on their part as well. A direct advertiser network is an excellent complement solution to your current advertiser feeds, one that offers higher margins for both you and the end user client. Higher margin solutions are more sustainable and make you more competitive.