Why do we breathe oxygen? how do we get energy out of our food?

We breath oxygen because it is needed to burn fuel [ sugars and fatty acids] in our cells to produce energy. Oxygen is also brought into the lungs via breathing; where it is transported by red blood cells to the entire body to be used to produce energy. The food we eat gives us protein, sugars, vitamins etc. for energy 

Out body breaks down food with enzymes and the glucose produced is burned to form energy measured in calories.

we breathe in oxygen to help our muscles function 

It explains why rocks appear in layers