1. Groupon is bringing offline local businesses online. The same way Google brought small advertisers/small businesses online.
2. Groupon could be a useful way for business to get some cash up front for future customers. That cash may help small businesses expand, instead of taking loans to expand.
3. Groupon cost of small businesses could be useful marketing cost since Groupon deal reaches out to many local customers.
Slightly old but interesting read on this: http://www.evanmiller.org/is-groupon-the-next-google.html
Further, this article itself mentions "Note massive competitors like Google, Facebook, Walmart, Opentable, etc. already doing their own versions of daily half-off deals". As people have said "imitation is the best form of flattery". So many Groupon clones show that there is a demand for such a service from both small businesses and consumer sides.
However, some of the accounting practices of Groupon and the way Groupon used recent investment money seems unusual and have raised a lot of eye-brows.