Google earns money just because someone shows you the ad. And if I'm not mistaken, when you click in some ad, the site showing it also earns money. So, basical

Google earns money just because someone shows you the ad. And if I'm not mistaken, when you click in some ad, the site showing it also earns money. So, basically, people spend money anyway, but you just "reward" the site when you click on it.