There is a video over on Youtube showing a lecture on sugar by Dr. Robert Lustig
It's very detailed, and long, but worth watching:
Sugar: The Bitter Truth
There is an excellent idea in there - that exercising to burn calories isn't very effective. You can't burn that many with exercise (unless you're training very intensely for multiple hours).
So why exercise?
Stress relief, for one - it lowers your stress hormones (cortisol). It also strongly influences other things people need (not all mentioned in the lecture):
- physical strength
- power (strength applies quickly)
- body composition (more muscle, less fat)
- cardiovascular health
And the aforementioned stress relief.
But burning calories takes a lot of steady exercise, and it's very easy to eat the extra calories to make up for it. Diet is the primary way to deal with excess body fat, and exercise supports it - it doesn't really drive it.
If you want to hear more with Dr. Robert Lustig, there is a short interview here done by Alec Baldwin, the actor:
Dr. Robert Lustig interview