Good coaches will make their expectations of conduct clear early on in the season. Even if they don't, you should make sure that your child understands your expectations before they ever go on the field.
Good sportsmanship is one of the most important lessons that we can learn from competition. Knowing that our success in sports is in learning to play the game with a certain attitude, rather than in whether we win or lose, and transferring that knowledge to other tasks and activities is an important part of the battle when it comes to learning to succeed.
It builds skills that all children and even some adults need to have for their own.