A large pitcher is 60 ounces in volume, and a pint glass is 16 ounces, so a pitcher holds 3.75 beers. This equals about one full glass each for three people, with some left over.
If you consider "a beer" to be a beer bottle, the average longneck bottle sold in stores is 12 ounces. So, you could serve five people a bottles' worth of beer from a pitcher.
Bars and restaurants like to price pitchers slightly cheaper per-ounce to encourage larger purchases. If you're sure your friends will drink it all, it's often not a bad deal.