Exercise is really important when dealing with anger and stress. When you are angry or stressed out your body is dumping a lot of hormones preparing you to either fight or run away. These can build up in your system, so to speak, and cause a lot of health problems. Exercise gives you a chance to burn these out of your system.
BTW, I have no medical experience and these are my metaphors for explaining what is happening. I am sure the reality is much more complex and scientific but doesn't really add anything to the post.
Your friend,