Given market wage W, a firm optimally hires the quantity of labor l*, where MRP_L = W. If Samsung hired less labor than l* so that MRP_L > W, it could increase its profits by hiring more because the firm’s benefit from that labor (MRP_L) would be greater than its cost (W). If, on the other hand, Samsung hired more labor than l*, it would be paying for labor with a benefit less than its cost, and could do better by reducing hiring. Only when MRP_L = W can Samsung do no better.