Food provides nutrients to our bodies. These nutrients will then provide energy for different activities we do daily, for us to grow, breath, keep warm and fight diseases etc. so without food we shall not grow, we have to energy to work, we would be prone to diseases etc and eventually die.
Our bodies constantly are in work to regulate its temperature. When you run it means there is increased heat and blood flow in the body. Therefore the body naturally will sweat to try and cool off the heat. A good thing it also helps us to detoxify (removal of toxic substances from the body)