I am no scientist but I try to stay up on literature related to food and exercise culture as well as the popular and scientific literature on exercise and diet. The most common conceptual model used in exercise is “calorie burning.” This is a mistake. We burn calories just sitting and it is biologically DIS-advantageous to burn through thousands of extra calories per day. by running on a stair stepper as a form of penance for eating a cheese burger. It is better to simply eat less to control calories.
Because of these factors, I propose a different model: exercise as poison. In ancient times intense physical exertion could leave a human being weak, helpless, and consequently close to death. The human body was designed to adapt to such stressors. In fact, through a process of rest and adaptation, it will build a tolerance for the next bout of trauma. This is similar to the Dread Pirate Roberts building up a tolerance for the fictional Iocane powder. Every time we exercise we should be trying to expose our body to a controlled amount of dangerous exertion so that we build up a tolerance to that trauma by resting and eating.
If our bodies are becoming stronger, more durable, and capable of exerting effort over longer periods of time without sustaining injury, then our exercise has been worthwhile. Or, if in your twilight years, exercise prevents you from losing strength over a 10 year period, it has been worthwhile.
The adaptations which actual exercise stimulates are awesome: metabolic change, new muscle tissue, better coordination in the posterior chain muscle, stronger connective tissue (bone, tendons, and ligaments), and a more powerful ticker. These benefits overlap but are not the same as the main adaptation which comes from sports and playing: the development of an activity specific skill set. Similarly, real exercise burns some calories, but that is not its point.