Groupon has over 500 highly trained inside sales reps working out of their Chicago headquarters.

These reps have established relationships with probably 100,000 [1] local businesses who are willing to spend money on online advertising.

Google's existing advertising products are all self-serve. (Large accounts get assigned a rep, but only ex post facto.)

This acquisition makes a ton of sense: Google gains a kick-ass AdWords sales team. With every Groupon sold, they would have the opportunity to ask the merchant "Can I tell you about our AdWords program?"

[1] My estimate: 730 days of deals, 3 deals per day, 50 cities = 109,500 deals. Granted, not all cities have side deals, and not all cities have been live for all 730 days, but they're closing in on 100,000 merchants fast.