Diff locks are designed to overcome the chief limitation of a standard open differential by essentially “locking” both wheels on an axle together as if on a common shaft. This forces both wheels to turn in unison, regardless of the traction available to either wheel individually.

When the differential is unlocked (open differential), it allows each wheel to rotate at different speeds, thus avoiding tire scuffing. An open  differential always provides the same torque to each of the two wheels, on that axle. So although the wheels can rotate at different speeds, they apply the same rotational force, even if one is entirely stationary, and the other spinning.

Diff locks can provide a significant traction advantage over an open differential, but only when the traction under each wheel differs significantly.