Greenhouse growers often add carbon dioxide to the air in the greenhouse What is the advantage of doing this?

All plants need carbon dioxide to complete photosynthesis. Mammals take in oxygen and breath out CO2. It is the exact opposite for plants. The stomata on the plants act as lungs for the CO2 to enter the plant. The plant then puts out oxygen into the air. Since plants need CO2 and they are putting out oxygen, extra CO2 in the air will hope the plant grow.