Since I brought up the question, it's probably pretty obvious that I would choose the latter answer as the correct one. I would say that the most important reason for forgiving someone is to bring our own minds and spirits to peace, for holding on to resentment and anger is one of the most common and destructive reasons for not having peace. And when we don't have peace, we affect our entire being--mind, body, and spirit--in very negative ways. Our stress levels go up, often causing physical problems like headaches or poor digestion or even higher blood pressure, depending on how long we hold on to the negative feelings. And as long as we're focusing on the negative aspects of what's happened, our spirits and our minds can't be free to soar and to enjoy life because the negative feelings drag our spirits down into the dirt, where there isn't a whole lot of soaring going on.
Forgiveness can be a great boon to the people being forgiven, especially if they sincerely regret something that they've done, mistakes that they've made. But there comes a point at which even the most contrite person will say, "Forget it--if you're too stubborn to forgive me, then I don't want to have anything to do with you." How many relationships have been ruined not by a careless action or a grave mistake, but by someone's unwillingness to forgive that action or error? It's an incredible waste of human energy to bear a grudge.
Is there someone in your life who could use your unconditional forgiveness? Then forgive them, now. You'll be doing a great thing for that other person or those other people, but more importantly you'll be doing something very important for yourself.