Answer:
The reason you eat is to ultimately create a molecule called ATP (adenosine triphosphate) so that your cells have the means to power themselves, and therefore you, along. And not incidentally, the reason you breathe is that oxygen is needed in order to get the maximal amount of cell energy from the precursors of the glucose molecules in that food.
The process human cells use to generate ATP is called cellular respiration. It results in the creation of 36 to 38 ATP per molecule of glucose. It consists of a series of stages, beginning in the cell cytoplasm and moving to the mitochondria, the "power plants" of eukaryotic cells. The two ATP-producing processes can be viewed as glycolysis (the anaerobic part) followed by aerobic respiration (the oxygen-requiring part).