The Law acts like a whip to the flesh, urging it on as men do a lazy sluggish ass. Even in the case of a spiritual man, inasmuch as he is still burdened with the weight of the flesh, the Law is a constant stimulus, pricking him forward when he would indulge in sloth. — John Calvin
For Calvin, sloth is a sign of spiritual immaturity. Hence, the Law is useful in prodding the slothful lazy sluggish asses forward (since they remain unmotivated within themselves and require external pressure).