As a Physical Education teacher I have actually had parents tell me my class is just "another recess". Yet every day I see students who display a stunning lack of basic coordination and balance. Not to mention the lack of endurance and strength and flexibility that is the norm rather than the exception.
Kids don't know what they need to know about keeping their bodies healthy unless someone teaches them! And not all kids are "natural" athletes. If you think kids can "figure it out" for themselves, just try teaching a cartwheel or a backwards roll to 2nd graders. The need for step by step instructions and demonstrations becomes immediately apparent.
Why limit a student's ability to use their body to its fullest potential by not helping them to completely understand its capacity?