Manners help us all coexist without disgusting, hurting and offending each other. When manners fail, people get very upset and try to enforce those social norms with laws. Law is not as gracious and forgiving as etiquette.
Also, good etiquette equips you with the correct response for all situations, so you're never at a loss for words and no one ever gets the upper hand on you. That reason alone is enough to be mannerly, but most people don't realise that.