Certainly, we can tick off the reasons why the Google/Groupon deal makes sense for Google:

It’s a growth strategy. As paid search inevitably matures, Google needs to stoke several new engines of growth to keep the topline growing. Groupon has scaled to $400 million in sales in two years, and clearly has lots of potential.
It’s a search strategy. What could be better to add to keyword results than a single, relevant deal of the day? You want relevant ads? Nothing’s better than a relevant, geo-targeted coupon on something you just searched for.
It’s a social strategy. The deal is an in-your-face move directed at Facebook, as it connects up the web of relationships to buying.
It’s a mobile strategy. As the world moves mobile, tablets proliferate, and Google’s newly federally approved AdMob ad network looks to 2011, a simple daily deal can make the most of location-aware shopping…