Why does exercising promote weight loss?

Burns & churns. Exercise burns calories while you are exercising. That is good. Exercising also churns your metabolism by cranking up the resting metabolism, burning calories at rest. Finally, daily exercising keeps you motivated and raises your self esteem.
Increases BMR. Our basal metabolic rate is increased with exercise and we burn more calories that we used to so there is weight loss with the same amount of food we eat if we exercise - it depends on the activity we choose the amount of weight loss.