My vote is the voice I get in democracy. I'm not going to use it to vote in someone I don't agree with on most issues, just because the other guy is worse. Me withholding that voice (or using it to vote for someone I know won't win, but who i genuinely believe in) is the way that I can be heard. It's my way of telling future politicians that if they support X (or something close to X) that they can get a portion of those who voted for C last election.
It's my way of sending a message that the next candidate shouldn't be a serial killer AND shouldn't be involved in tax fraud. It's my call for someone who better represents me and my beliefs.