Realistically, it does very little to keep you thin on its own. Try to outrun a single donut and you've got your first 5k of the day. The benefits of exercise are a fitter body and a fitter mind, really. You feel great in every way after (and this is coming from someone with a sore upper back...). I'd recommend exercise for just the reason you mentioned, it makes you feel fantastic, and it's nice to be able to do things without even thinking and then realize that some people struggle to do them (stairs, carrying heavy groceries or laundry around, etc, just simple everyday things).