1) You get cut a check for a substantial amount of money, potentially weeks or months in advance of having to provide the good/service which justifies the money.

2) Exposure to a new audience, who you could ideally:

a) convert into recurring customers

b) upsell into more lucrative services

c) use as social proof / infection vectors for recruiting more customers

3) Did I mention the cash flow thing? Seriously. I think that is huge. Groupon is in the business of calling people on the phone and promising to give them a lot of money. And they do it.