The main disadvantage of an "open" differential is that the usable torque is restricted by the wheel with the least traction. A differential lock literally locks out the differential action and forces torque to be split equally between each wheel (or each axle)for maximum traction. In a rear-wheel drive vehcile, for instance, locking the rear differential locks the left and right rear wheels together so that both receive equal torque.
In a full-time or permanent four-wheel drive wehcile, locking the center differential locks the front and rear drive-shafts together, sending equal torque to the front and rear axles. While locking a differential helps increase traction, it makes steering more difficult on high-traction surfaces because it tends to force the vehcile to travel in a straight line.