We breathe an element in the air called oxygen, it's what the human body absolutely needs in order to live. After we process it, what we exhale is called carbon dioxide, think of it like exhaust after fuel has been burnt. Burning coal produces large amounts of it too, the body tosses it out because it's useless to it after and unhealthy to let it pile up.

Green plants, on the other hand, are capable of a process called photosynthesis. During the day, it has the property to take in carbon dioxide, water, and sunlight, turning the output into oxygen. In other words, it's capable of changing exhaust back into fuel. That's why they're so vital to life, and given that we breathe out when we talk, the symbiotic relationship is beneficial to plant and person alike.

[so how's that for someone centuries behind modern science times...]