I've been in the past told by Google that they do pay for their ads just as well as the advertisers.
Haven't seen one in a loooong time on my sites.
As far as I understood they are CPM ads, not CPC. So you get paid just for showing them (a little bit). But it's better than nothing and if they show up I guess it's a sign there's little advertiser interest for your audience/yourself (as a viewer of ads).