It teaches them how to put themselves in others' shoes.
So that they can learn Humility, Compassion, Respect, Empathy, Tolerance and the road to Wisdom and Fortitude. There are a lot of reasons but those are the basics.
helps clean up the beer cans and vomit from the night before
I as a high school student, think that it teaches students how one little action can affect the community, learn how helping others is nice, and sometimes you don't need to get payed for doing something but doing it because of the good of your. something like that :D
Saves on taxes--you don't actually have to pay people to do what needs to be done.