Physical exercise contributes to keeping your body healthy, but how can it provide stress relief?
Many times stress is caused by having too much energy and no outlet - the mind then decides to spend that energy on worrying over something and converts it into stress. In this case, physical exercise channels that energy into a constructive activity, and exercise becomes something akin to meditation which helps prune away dangling thoughts.
In case there is a lack of energy (maybe if stress lasted too long and resulted in a state of depression), exercise can help build up motivation and focus.
Exercise is about reconnecting with your body, becoming more aware of your rhythms, limitation and capabilities. It puts your mind back into HERE and NOW, this point in space-time where you are yourself (and not the things you stress about).
Exercise also makes other forms of stress relief easier: after exercise it is easier to relax, meditate, or at least have a good night's sleep. Mens sana in corpore sano - it is much more wholesome, natural and easier to have a healthy mind in a healthy body than to have only either one.