Can someone explain spectator sports?

There is no real need to describe how important spectator sports are in the U.S. (and elsewhere). But what explains their appeal? Do they serve a purpose?

I’m not asking why soccer is fun to play – that’s an easy one. I’m interested in why we (humans) seem to identify with a team and enjoy watching that team play and win. Grown men walk around wearing shirts with other guys’ names on them. They seem to genuinely care whether a group of guys wearing costumes and throwing/kicking a ball wins/loses to the point of it affecting their mood the next day, or even rioting when their emotions get out of hand.

Is it just pure tribalism? Are we fulfilling a need to create an “us” and “them”? Or does sport create or foster these emotions?