Why does our body need nutrients?

Your body requires nutrients in order for it to function properly. In living, we consume energy which keeps our organs functioning, brains thinking and all that other life stuff. By expending energy, our body is using up the energy it gets from the nutrients we consume when we eat (carbs, proteins, fats) and even that which we gain spending time in the sun (Vitamin D).

Mal-nourishment (lack of nutrition) leads to all sorts of lovely diseases such as rickets and kwashiorkor, because our bodies are no longer able to keep themselves healthy.