Proverbs 25:28

A person without self-control is like a city with broken-down walls

In anciet times, a city whose walls were broken down was extremely vulnerable. They were vulnerable to attack but also to wild animals. An athlete or a team without self control is also extremly vulnerable to their opponent and also to negative consequences (from the law or others). This is why being disciplined is important. If you are not disciplined, you will not reach your full potential.