Heat is that propels the physical movements in the body apart from the enzymatic reactions. In a heated area, blood flow is more. Even irritation is a form of heat. Whenever there is more blood flow, there is more supply of nutrients and more removal of waste and toxins. All movements in the body accelerate blood flow.
Yoga is that which produce the heat that drives the flow of blood.
Any part if unused deteriorates with time. This applies to body parts too. If any region does not get proper exercise would suffer from diseases. Medical Researchers and Doctors advocate exercise, but many do not understand its real benefit. For example, they know that exercise and estrogen levels in women are related.