Your body needs to be active to be healthy. A lack of activity means the fuel (food) you put into it isn’t be used, so your body stores it for later in the form of fat. When you are more active, your body burns its fuel, partially fat and partially what you just ate, in order to supply the energy it needs to keep exercising. Beyond that, keeping your body in shape means your metabolism is more efficient. A healthy body is able to stay healthy more easily. Without activity/exercise, your body slows down and fat starts to pile up.