Even if you eat a reasonably good diet, exercise still benefits by raising your metabolism and boosting cardiovascular capacity.
Studies have shown that exercise is helpful in combatting depression thanks to the endorphine hit that most physically fit people experience after working out. It's a great way of dealing with stress and anxiety too. If someone p#sses me off I take it out on a running machine or in the swimming pool rather than bottling it up.
If I don't exercise for a few days I begin to get grumpy and irritable....