It works for the AdWords advertisers because their ads go everywhere. Not only will they find themselves listed in Google’s search which gets gazillions of hits per day, up front without working as much for SEO and waiting so much.
Their ads can reach any website that deals with anything similar to what they’re trying to sell. Now you must realize they could never pull of such great advertising by themselves. And that brings us to the thing that makes Google’s AdSense a publisher’s best friend.
It comes from the fact that the ads are contextual, that they somehow related to the keywords you deal with on your page. Because people or on your site, which deals with a certain topic, you already know they’re interested in that topic.
But, hey, wait a minute, Google knows some companies which want to sell your visitors something related to their topic. Google wants your visitors, you want Google’s advertisers and the visitors just want to buy stuff. And that is the essence of what makes AdSense a great deal for everybody.