Formerly, people walked more and their work used to be physically harder: miner, woodcutter, fisherman, farmer, rancher,… Currently, science and technology have advanced much, so people use more the means of transport, those works are less common and technology performs partially or totally those activities. That change has occurred in relatively few years, so the body has not had time to adapt genetically. In addition, in the last years, some technological advances (computers, mobile phones,…) have made that life (mainly of the youngest) is even more sedentary.
Therefore, there is currently a growing pandemic (world disease) of preventable (especially) or treatable diseases through an appropriate lifestyle: diseases for excess of body fat (overweight or obesity), of bones, of heart, etc.
Physical exercise gives room to a correct and natural state of the body, and is necessary for health: correct functioning of the body, correct body structure and composition, correct psychological state, correct growth and development, prevention and treatment of diseases, deceleration of aging, etc. And I speak of “correct” (not of “better”) because (although it is evidently better) sedentariness and its consequences give room to an incorrect and unnatural state of the body: malfunction, etc.