It's a pretty smooth move on his part. A guy who buys drinks for his friends shows he has money and is generous with it, both DHV's. Buying and pouring the group puts him as the alpha male and as the "provider" for the group.
3:7 are good odds for a situation like that; I'm sure he wouldn't buy pitchers of beer for a table of almost all guys and just a few girls-- it'd be a wasted investment. Girls in general drink less beer than guys, and get drunk quicker. By buying and pouring drinks for everyone, they're somewhat obligated to drink it, even if they normally do not drink beer.
~Antithesis