Once you get to high school, other than the one or two years of Physical Education that is interspersed with health and Driver's Education, few of the students actually exercise. So a small portion participate in strenuous sports clubs that already condition them, but the majority do not. Therefore, participate in P.E. is a helpful way for them to get out there and exercise. Also, it provides a sound basis for the students who may teach others how to play, such as their children. Another reason is that it provides a background in some of the sports. For example, when you are older, you will likely be invited to hang out with your friends for recreational activities that does not involve clubbing. You may play baseball, softball, floor hockey, basketball, etc. It would be no fun if you didn't know how to play or you just plain sucked. P.E. then helps students build up on that skill.
Speaking of building, one can also build up their muscles during the critical part of the teenager's growing period. Many students who, once they get to high school, can't even make two push-ups. That is a defect of many, since such an exercise is extremely beneficial for cardiovascular reasons. It also strengthens the muscles in the arm, and increases endurance and stamina.
Physical Education also provides an outlet for students to let loose their frustrations, particularly stress. Many actually have fun while playing the assigned sport, even if they are playing badly. By moving those muscles, the body can relax while it builds up muscle strength and endurance, so the body gets double the benefit.
If you want to see the results of those who didn't bother with the benefits that physical education, or even physical exercise in general gives, then go outside and look at those who are in few terms fit, but overall overweight