Why is there a mandatory element of exercise and physical training?

You can with a little thought find 3 reasons for this;

1. Physical exercise has been shown to improve mood, combat chronic disease, manage your weight, improve motivational and energy level, lead to better sleep and improves cognitive function. 
2. A good physical preparedness will prepare you to be in the best possible condition to deal with the unexpected situations you may face in life whether that be severe trauma or chemotherapy,  being able to help others after an earthquake or tsunami or simply to run and catch the bus that will get you to your job interview. Preparing for the unexpected before it occurs is always preferable to wishing you had done more to prepare later. 
3. It is n the best interests of the community and yourself that you live a long and healthy life, do not squander or throw away the remarkable body and life you have been lucky enough to have. Allowing your body to deteriorate is a prolonged form of suicide you should be circumspect of the way you treat it.