Poop is waste. It is the part of the food that remains after all useful as nutrients are utilized. Food is a kind of human “fuel” in the same way as gasoline in cars. Car exhaust pipe will exhaust gas and the human butt will poo.
While your body can not utilize poop substances, plants instead can. That is why, for example, a cow or a chicken poop is a good fertilizer and helps plants to grow well. The plants are then eaten and become poo again. This is how the natural life cycle works.