State the usefulness of Newton’s third law of motion.

Newton's first law and second law does not explain how the force acts on the object. This question is answered by Newton's third law, which states —

"To every action there is always an equal and opposite reaction".

Example — While moving on the ground, we exert a force by our feet to push the ground backwards, the ground exerts a force of the same magnitude on our feet forward which makes us to move forward.

Hence, the force exerted by our feet on the ground is the force of action and the force exerted by the ground on our feet is the force of reaction.