I'm still not 100% sure on the point. So I get an email once a day offering me a random discount on a random product. I have to decide whether I want to buy that product. And then, when it comes time to do my weekly grocery shopping I have to remember all the products which I've already signed up for and collect 'em all.

This seems to have saved me a little bit of money at the cost of a bunch of mental effort.

edit: Mind you I've never been persuaded to use Groupon either, so maybe it's just me.