When it comes to traction in off-road situations, differentials play a key role. In general, the differential in a vehicle's axle allows the outer wheel to rotate more quickly when the vehicle turns a corner. The outer wheel travels about 30 feet farther than the inner wheel in a 360-degree turn. A 4WD vehicle has one diff in each axle.
Locking differentials (or diff locks) are the only cure for the shortcoming of differentials sometimes sending torque to the "wrong" wheel. Diff locks disable the differential's ability to distribute torque equally and its ability to allow the wheels to rotate at different speeds. In some ways a diff lock is the ultimate traction device. Since, all wheels rotate at the same speed when engaged, a vehicle with locks on all axles has traction if just one wheel does.
By disabling (or locking) the differential, steering becomes very difficult and the wheel with the most traction will get the most torque, as much as 100%. This guarantees that any wheel with traction will receive enough torque to move the car. In sum, diff locks will keep you going and keep you from getting stuck.