the atmosphere is made up of 21 per cent of oxygen and the rest is mainly nitrogen,if the atmosphere was high in oxygen we would over oxygenate our body leading us to death,but since our lungs do not absorb nitrogen, when breathing,we get the right level of oxygen. so basically in order to have oxygen,we need nitrogen.
also, we need nitrogen to make proteins in our bodies( muscle,skin,DNA,etc) in the sense that our bodies need nitrogen in the amino acids from food containing protein to produce more amino acids
