As the other guy said, it's simply for vote/rating purposes.
You can actually de-upvote yourself or any post you have rated before (there is no limit on how many times you can vote than cancel later, you just can't stack the vote to be more than 1).
You simply need to click the same arrow again, for example everyone's post starts with +1, by pressing the up arrow on your own post you undo the upvote and becomes +0, same thing applies to downvotes and any post you have rated.
If you instead press the other side of the arrow, it will cancel your original vote and rate it to the other side of the spectrum, for example for my +1 post if I press the down arrow instead of going to 0, it will become -1 as it cancels the +1 THEN downvote it by another 1.
If you get what I mean here.