Grease gets a high position in the list of greatest elements for energy production, only surpassed by nuclear fission and nuclear decay, hydrogen and hydrocarbon based energies. It has a tremendous energy density, more than explosives like TNT.
This means something important, because it's very difficult to get rid of it when the body accumulates considerable quantities. Perhaps the word is not difficult but slow, since it would need time and patience. Any diet should include the necessary nutrients and be gentle with the organism. Otherwise there's chance to inconveniences for health.
Biochemicals like fat release energy when their molecular structure get oxidized by enzymes. A major source of energy in Nature's kingdom, complex and efficient as always. However the issue is the bad use, caused by the misunderstanding of its properties and what Nature wants in energy optimization.
You could understand what fat can do running 5 kilometers. If your weight is 70 kilograms, to move them 5000 meters, the kilocalories counter stays under 400 kcal. Running! Less than half a glass of oil or a bit of cheese. A marvelous OPTIMIZATION.