Also there are more "branding" ads which are placed just to get the company's logo in your face and don't necessarily need a click to do their job.

If I want to advertise my new Windex(tm) flavored Mountain Dew[1] then all I need is people to see it. They're not likely to buy it from a click, but I want them to remember it next time they are at the grocery store, and I'll pay to shove it in their browser.

[1] Not currently available in retail stores