Moving the Body not only moves the muscles but it also makes the heart work more; more blood is pumped and moved about the body, the breathing muscles are exercised and lungs are trained to increase their capacity. This is why we use movement as exercise.
Something else also happens when we move the body and exercise; Qi/energy also activates and moves. When we consider the most prevalent type of exercise such as running/jogging and lifting weights/resistance training, the emphasis here is on the movement of the heart and lungs, blood and fluids, tissues, joints and muscles. Any Qi/energy movements are purely incidental.
Qigong is the Art of carefully tuning into the body and moving the body in such a way that it nurtures Harmonic Qi movement in the body and strengthens the channel/meridian connections. This subtle energy is the missing piece that connects the body as a functioning, Living Whole, as opposed to separate components of 'hardware' joined together. Once you learn to 'Tune in' to the secret movements happening behind the scenes, Movement is never the same again, and something as simple as moving your body becomes a pleasure!