PPP Direct was launched as part of a number of big new features offered by PayPerPost to it's Posties. With this set-up, Advertisers can directly approach the Posties, who can then write special blog posts offered to them. A widget placed on the blog will be clicked by the Advertisers, and then the Posties will be given an offer. This way, you make money at your own terms.
Some competitors charge 50-100% markup on your post, while PayPerPost charges only 35% for their self service model. A mark-up is understandable since the competitor have to pay for matchmaking, customer service processes, and tax processes. These are the ones that costs real money. However, at PayPerPost, they have erased the need for a middleman. Most of the details will be placed on your widget and from there, it's up to the Postie if the offer made by the Advertiser is good enough, or not. Hence the lesser charge, and you get to keep and earn more money from your efforts.
This doesn't mean, though, that PayPerPost has totally abandoned the Postie (as well as the Advertiser) when it comes to PPP Direct. That's why they still maintain a 10% service fee, which is undobtedly lower compared to other competitors. PayPerPost is there to provide a steady source of security for both Posties and Advertisers.