"In the face of war, social injustice, religious conflict, corporate greed, what difference can one person make? Why should one person even try to make a difference? The odds are so overwhelming. It's pointless."
Frankly, that doesn't matter. We do what is right simply because that is what it is—right.
I don't refrain from striking another person because I expect that action will lead to the end of violence, but because not hitting another person is the right thing to do. I don't refrain from lying to my spouse because I think my actions will convince everyone to be honest, but because being truthful in any relationship is the right way to behave. I don't refrain from cheating a customer because I know he'll never find out and I don't like the color of his skin, but because treating all people equally and with respect is the right way to interact with others.
I refrain from wrongdoing because even if no one would have known of the wrong I did and even if my right actions have absolutely no effect on others, doing what I might have done would have simply been wrong.