Not only is exercise good for your dog physically, it's also good for him mentally. When I worked at the vet clinic, we would often see younger dogs for behavioural issues (things like chewing and eating things they shouldn't have, and then having to go in surgically and retrieve those things) and it was often tied to a lack of exercise or play. Dogs are like us, they get bored if they aren't stimulated. And a bored dog can be a destructive dog.