We breathe out carbon dioxide because it is a waste product produced by our cells during cellular respiration. Our cells use oxygen to break down nutrients, such as sugars from food, to create energy needed for bodily functions. This process generates carbon dioxide as a byproduct. Carbon dioxide is transported by the blood to the lungs, where it is released when we exhale. Removing carbon dioxide is essential to maintain blood pH and allow the blood to carry more oxygen. If carbon dioxide were not expelled, it would accumulate and reduce oxygen capacity, impairing cell function. This exchange is a vital part of how our bodies generate energy and maintain homeostasis.

In summary, breathing out carbon dioxide is the body's way of getting rid of this metabolic waste to keep cellular processes running efficiently and to maintain proper blood chemistry.