Here's why:

* Photosynthesis: Plants use sunlight, water, and carbon dioxide (CO2) to create their own food (sugars) and release oxygen as a byproduct. This process is vital for life on Earth.

* Respiration: Animals, including humans, breathe in oxygen and use it to break down sugars for energy, releasing carbon dioxide as a byproduct.

This continuous cycle of photosynthesis and respiration keeps the balance of oxygen and carbon dioxide in the atmosphere.