Physical (body) and cognitive (brain) development go hand-in-hand. While this continues for life, this relationship is most critical at a young age. When kids are active, their brain develops, allowing for new types of development.
Play based activity that requires a high degree of sensory input (sight, sound, touch, etc.) helps develop a broadarray of skills that make physical activity more enjoyable later in life.
Strengthened muscles and bones to help with optimum growth.