A spinning front tire six inches off the ground can strand a truck that is otherwise perfectly capable. The engine still makes power, and the transmission still sends it somewhere. But an open differential sends all of it to the wheel with the least resistance, which in this case is the one doing nothing useful at all.
That's the exact moment a locking differential earns its keep. Lock the axle, and both wheels turn at the same speed no matter what each one is touching. That's why Jeep, Ford, and Toyota all still bolt them to their serious off-road trims.
