Fitness keeps you healthy and helps you avoid a lot of disorders, such as heart diseases, and obesity.
Keeping yourself fit allows you to enjoy life it's full. You don't have to worry about a lot of problems that overweight or unhealthy people have.
Your body remains in shape, you look more attractive and your face glows, this also boosts your confidence in yourself. Thus further boosting your mental fitness.
Fitness also impacts your mood and you become happier.
Fitness also impacts your social life, you are more in control and more confident, this helps you impress the people around you.