Living things breathe to live.
We need this for fulfilling the cycle in which plants give us oxygen and we give plants carbon dioxide. If you keep a plant and cover it it will slowly wither, different organisms use different organs to breathe.
All living things need oxygen. The oxygen reacts (or combines) with food you eat to make energy. You need energy for everything you do: for moving, eating, growing, fixing injuries, keeping warm, and even thinking. You can only live for about three minutes without fresh oxygen.