Dealers probably have bulk rates figured out, but the listed price for listing a Cargurus is $99 after the car sells. Like I said, dealers probably have something worked out, but that's the main source of revenue for sites like this. When I sold cars, we used to have a cars.com rep come to our office every month and bring us donuts. We posted our inventory on their website, and as such, we had a rep assigned to us. He'd help with our listings, review our metrics with us, etc.
It's both advantageous for the buyer and the seller- the seller gets their cars in front of more eyes and the buyer gets things like Cargurus Instant Market Value tool and prices that are really as low as dealers can go because they are openly competing on the same platform. If you go into a dealership on a car you found on Cargurus with a "Great Deal" tag with the intent to negotiate even further, you may be in for a rough go of it. That said, you don't have to negotiate much anymore, so car buying can be a much more quick and painless experience.