Question

why the proteins are so useful to burn fat and build muscles?

Answer 1

well to say it that way is technically incorrect.
I think the easy way to say it would be that your body stores extra energy as fat. This comes from overeating primarily. Proteins are useful to build muscle because they are the primary element of muscles. All motion in the body is controlled by protein being triggered by atp. Theoretically if you have more protein then you burn more energy.
tl;dr if you eat to much you get to fat.

Answer 2

Sometimes your body stores extra unneeded fats that you should sometimes need to burn off.Proteins are useful to build muscle because they are the primary element of muscles. burn off.