Fat is your body's main source of energy. That is why the body stores it in such great amounts from the food we eat. Fat breakdown is slow and thus provides a better fuel for long term at rest activities like standing, sitting ,and routine walking.
FAT is the primary source of energy at rest, but as the intensity of your activity increases it shifts from fats to carbs.
Food is primary source of energy....but energy is released when food is oxidized.