I'm surprised there are still any of those around period. It was very popular once upon a time.
It worked like this: It's March of 2000. You are in the bizdev department of idiotic.com, a free portal site employing the entire population of a small Northern California city. Your site is supported by venture capital dollars and advertising revenue from the ads you sell on your pages. Because it's March of 2000, you negotiate a ridiculous per-view price, with a bonus for clickthroughs. Your eyes light up with little dollar signs when you realize that, with your current hit rates (and some "projected future growth" numbers which fail to take into account that the world only contains six billion people), you realize that by 2006 you'll only be losing three gazillion dollars a quarter.
In March of 2000 dotcom terms, this is defined as "success," but you're not satisfied. You're determined to take the industry by storm by becoming the first free portal site on track to lose only TWO gazillion dollars a quarter by 2003. So you take some of that free money the nice venture capitalist gave you out of the goodness of his heart and you make a deal with this little company that pays users when they visit certain sites. The little company pays users significantly less than the exorbitant amount you charge advertisers, so you can afford to pay a company which in turn pays people to look at the ads you're being paid to display.
It's good for everyone... well, everyone except your advertiser, who gets hit with an entirely unexpected 150 bazillion dollar bill for page views, most of which were college kids in search of money for beer and smokes who found ways to automatically surf to sites on the list over and over and over with nobody actually looking at the screen. And for his 150 bazillion dollar advertising investment, your advertiser saw 173 clickthroughs and 8 purchases. Net ROI: approximately negative 150 bazillion dollars, give or take the value of 8 widgets.