Why is it essential to set hand brakes on cars of a train laid up on a grade?

Setting hand brakes on cars of a train laid up on a grade is essential primarily to prevent accidental movement of the train. When a train is parked on a slope, the gravitational force can cause the cars to roll if they are not secured. Hand brakes provide a mechanical means to ensure that the cars remain stationary, thereby increasing safety for both personnel and equipment.

While the other choices address various aspects of train operation and management, they are not the core reason for setting hand brakes on a grade. For example, while assisting in loading and unloading is important, it does not specifically address the need for security on an incline. Similarly, reducing wear on brakes and assisting in acceleration are concerns of proper train operation but do not relate to the immediate risk of movement associated with being parked on a grade. Hence, the primary function of the hand brakes in this scenario is to secure the train against any unintended rolling.