Exercise for exercise. Exercise for a long, healthy, active live.
Exercise for functional capacity, mood stability, improved attention and learning.
Exercise to prevent disease. Exercise to improve your quality of life.
Exercise because not exercising increases your risk of all causes of death and all disease states.
If you exercise only because you think it will help you lose weight, and exercise is your only weight loss strategy, chances are you won't lose weight. And then you will think exercise failed you. You will be wrong. Exercise is the sine qua non of health; it will never fail you. if you do it for the right reasons. There are many reasons, see the first paragraph and the research studies below.
If you want to lose weight, change your eating patterns. Many people need to lose weight.
If you want to be healthy, exercise - daily.. Everyone, regardless of their weight status, needs to do this.
(NB. this was posted a few weeks later http://www.vox.com/2016/4/28/11518804/weight-loss-exercise-myth-burn-calories)
Janssen, I., & LeBlanc, A. G. (2010). Review Systematic review of the health benefits of physical activity and fitness in school-aged children and youth. International Journal of Behavioral nutrition and physical activity, 7(40), 1-16.
Nocon, M., Hiemann, T., Müller-Riemenschneider, F., Thalau, F., Roll, S., & Willich, S. N. (2008). Association of physical activity with all-cause and cardiovascular mortality: a systematic review and meta-analysis. European Journal of Cardiovascular Prevention & Rehabilitation, 15(3), 239-246.
Penedo, F. J., & Dahn, J. R. (2005a). Exercise and well-being: a review of mental and physical health benefits associated with physical activity. Current opinion in psychiatry, 18(2), 189-193.
Warburton, D. E., Nicol, C. W., & Bredin, S. S. (2006). Health benefits of physical activity: the evidence. Canadian medical association journal, 174(6), 801-809.