Why do you need to be fit?

The fitter and healthier you are, the longer you will live. Your body's immune system will be protected from more diseases, you will recover from injuries faster, and you will be happier (the bodies of healthy people produce more endorphins, which are chemicals that produce happiness in the brain).

Being healthy, fit and strong also improves your confidence and your appearance.
Because if you don't you'll eventually get sick.