Basically, if this is the project I think it is, all they're doing is letting you act as your own advertiser, with the ad that you're running being an empty pixel pattern.
Good call. Basically, they can turn users with a PREF cookie into additional bidders for ad auctions on their network (and others, maybe?). $1-3 isn't much, but if you pool a large number of users' money, you can both drive auction prices to increase your revenue, and drive users to contribute more when their "contribution" stops removing ads.
Protection rackets aren't exactly a new way to make money. People don't like ads, and Google serves many of the ads, so they can shake people down, and use the extra money to make their competitors' lives harder. Meanwhile, their tracking business is unaffected.
