Many planets have atmospheres of some gaseous mixture. This mixture of gases is exactly what humans, animals, trees and plants need to exist. Without air, there could be no life on earth. Air shields the earth from harmful solar rays and objects from outer space hitting earth. Air traps heat from the sun, giving earth enough heat to support life.
Air also makes it possible for sound to travel to our ears.
Air has weight and pressure. The weight of air pressing down on us produces air pressure. An atmosphere is a pressure of 14.7 pounds per square inch of surface. That's at sea level. 20,000 feet is 6.8 psi.