What use does the body have for the nitrogen in the air we breathe which is more than the oxygen?

Oxygen (O2) makes up about 21% of the atmosphere while nitrogen (N2) makes up about 78%. The nitrogen we breath is not in a form that can be used by our bodies, so after it is inhaled it is simply exhaled again as nitrogen. The balance of gases in the atmosphere is crucial to life. If oxygen comprised less than 15%, fires would not burn. If oxygen comprised more than 25%, wet organic material would burn freely. Still, nitrogen from the atmosphere is "fixed" in the soils where it can be assimilated by plants through roots, then eaten by animals, so the human body receives nitrogen that it can utilize, less directly, through food sources. Eventually some of the nitrogen in the atmosphere does enter our bodies although it is not directly by breathing, it is by eating. The process of nitrogen fixation is complex and difficult, and occurs by specialized bacteria in soils, or through the intense heat of lightning in the air after which the fixed nitrogen is washed into the soil by rainfall.