If someone (a spouse, parent, friend, boyfriend/girlfriend) has hurt you very deeply, why is the advice most of the time to "walk away", "let it go", "have no contact with the person"?

It seems counterintuitive that someone that does something that hurts you should be able to "get away with it" if you don't tell them what they did and how that affected you. Granted, if the person is dead, sometimes writing a letter and not mailing it is a good idea, but what if the person is still alive and kicking? Why shouldn't they hear/read what the person who was hurt is feeling? Yeah, I know that it may not make a difference to the one that hurt you, but I think it's better to let it out.