You can be blackmail even if you're a good person. Imagine you're getting famous, or you have been elected to a position of power. You meet new people and you get invited to parties. Everything is fine, you're having a good time when suddently, you feel light headed. You pass out. You wake up in a bed, naked, with some kids, also naked. One is griding on you while someone is filming. Now they have you by the balls: "Do what we tell you or your little escapade ends up on the internet." And if you don't bend to their will, they can just kill you.