From talking with a "bar designer": Every bar should have a choke point, that is a place where customers have to squeeze past many other customers. This isn't an annoyance, but an essential part of the experience.
Big empty rooms kill the buzz. Two different spots with a choke point in between make it rock. Think inside (doorway) outside, or lounge (doorway) kitchen. Even better when everyone has to make the pilgrammage between (ie, toilets, drinks ,etc).
I imagine it's for the same reasons. In my single days, I always stood/sat at the bar. You're guaranteed a turnover of people to talk to.