So our bodies can be nourished and strengthened and do the good that we need. So we can be energized and have good health. So we can live for a long time. "you don't have a soul. You are a soul. You have a body".-C.S Lewis. Your body is the shell to our souls, take care of it! "do unto your body as you would have it do unto you"-Jamba juice cup
In simple terms; it's because you have to stay healthy. Failing health is not good as it prevents you from doing what you have to do like work or even play. When you eat healthy, your body has the energy to do all these things and more.