The only way it makes sense is if they are able to get large volume discounts on whatever they are trying to sell.
Just running some #s here. Local clothing ship buys jewelry for $5/piece and marks it up to $15. Groupon takes 75% of the $15. Groupon takes $11.25 out of the $15 and you just lost $.25 on the entire deal (not including expenses to find that piece).
Sure that same retailer could mark the piece up to $20 but groupon takes $15 of that and you break even. A $5 piece probably couldnt be sold for anything higher than $20...
I will agree that it generates foot traffic but most smaller businesses I have talked to who have done a Groupon or thought about it have had negative experiences or choose not to do it because of a scenario similar to the one above I portrayed.
Just my .02