Respiratory tract
The airways' job is to bring air to the lungs, so it can get to the alveoli.  At the alveoli, a very important process called "gas exchange" takes place.  Oxygen from the air breathed into the lungs goes into the blood.  The blood can then carry oxygen to every part of the body through the bloodstream.  During gas exchange, carbon dioxide goes from the blood into the lungs, so it can be breathed out.  If this exchange of gases did not happen, the body would not get enough oxygen to stay alive, and would also get poisoned by carbon dioxide.