concessional speech

Has an individual in a congressional or gubernatoral election ever officially conceded to the apparent winner and found out later that he was the winner after all?
And secondly, does concession have any legal significance?

Thirdly, why is it almost always given by the loser? Is it just tradition? or good sportsmanship or what?

A concession speech has no legal significance; all that matters is the votes.

As for why it’s given by the loser, well, that’s what “concede” means.

The loser gives a concession speech to tell people that there are no hard feelings and all that.

Even though there usually is. But it makes us feel better.