Next to your own question or answer, there are up and downvote arrows:
Of course, you can't give reputation to yourself:
Why are the buttons even there?
(Might be good UI design, with the consistent presence of buttons. Just wondering.)
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Consistency, I guess. Whenever I see a post in the LQ queue (this queue doesn't show the voting arrows, only the net post score), the question feels naked to me (even after doing a lot of reviewing -- I'm still not used to it).
For new users, the score with arrows near it makes it clear that the number is the post score. If they try to vote for their own post, they are told that they can't. They'll remember that :)