I'm not in advertising, but I'd guess "Volume".
Google charges a fraction of a cent per "click". Adds up.
Company that's advertising only needs to sell one widget, and that covers a whole bunch of "clicks".
Add in the fact that the advertising is directed at you, personally, based on your surfing habits, and I suppose that it's relatively effective. Well, maybe not on you, but on some people.
I, too, would need to think pretty hard as to whether I bought something based on internet advertising. I probably have, but not much.
I’ll google an item and when I find it I’ll manually type in the address rather than use google as the portal to get to the retail site.
That said, I have never bought from a google click.