Who knows how the tradition got started, but at some point somebody thought that having a bunch of people together was a good opportunity to give people the chance to publicly show respect for the country they live in. Maybe it started back in Roman times when they got together to watch the gladiator playoffs.
Personally, I think it's a great idea. Most people never get to recite the pledge of allegiance or sing the national anthem except when they go to sporting events. Where else would you hear the national anthem today? Most TV stations go 24 hours a day now, so they don't play it at 12:00 midnight like they used to.