It is 100% about positioning.
If you've placed it well (the google heatmap is pretty accurate in my experience) then, if there's an ad which is of interest, it'll get clicked. You'll get the exit money and with a bit of luck the advertiser will get a conversion.
Absolutely, most of the time an AdSense click represents an exit route. That may be a content decision (this site is not for me but this ad looks interesting) or a lead decision (they've read your Widget review and now want to go buy one online).