Once a big event is over, everyone involved from planning to executing is exhausted. Even if an event is energizing and goes off without a hitch, tiredness is inevitable.
But many times, once an event is over there's still work to be done. Cleaning, tear down, returning used goods, all the little tasks need to get done. It's tempting to skip out early, after all, you helped plan this thing from the start!
If you do skip out on those last hours of work, no one may fault you. But, you'd missing one of the prime moments to grow closer with your teammates. Nothing bonds people together more than knowing you're all pushing through exhaustion to finish a project. It shows loyalty, develops trust, and strengthens relationships.