It teaches us the benefits of hard work. Even when you lose after practicing or working out so hard you have improved yourself and that almost always pays off somehow. What is wrong is sportsmanship or just plain courtesy to your opposition. No where in life will strutting around like a fool, shouting & pointing when you've won ever serve a purpose other then to make you look like an ass.
Competition is what drives people to do better and work harder, but it is emphasized on quite strongly it seems unneeded. Today almost everything we do is a competition with someone. You play a video game to get the best scores, or get the best gear, at work you try to be the best worker. It is everywhere.
Even in terms of evolution there is disagreement about whether competition or cooperation got us to this point. And in every day life, we are way too focused on outdoing the other person. We should be emphasizing doing our own personal best and also enjoying life rather than always trying to be superior to another person.