Oxygen is used in the respiration of humans and other animals. Humans need a continuous supply of oxygen for cellular respiration, and they must get rid of excess carbon dioxide, the poisonous waste product of this process. Gas exchange supports the supports this cellular respiration by constantly supplying oxygen and removing carbon dioxide. The oxygen we need is derived from the Earth's atmosphere, which is 21% oxygen. This oxygen in the air is exchanged in the body by the respiratory surface. In humans the alveoli in the lungs serve as the surface for gas exchange.
Oxygen is a vital gas required for the existence of living things.we inhale oxygen so that it  moves to the tissues for the glucose to get oxidized to produce energy and as a by product co2 is also formed. nitrogen is not a highly reactive gas .if the atmosphere was only of oxygen then the world would have burnt down billions of years to prevent that oxygen is only 21% in the atmosphere