A run of the mill rear differential has spider gears allowing one wheel to travel further when going around a corner. Also. On slippery surfaces the wheel with least resistance will spin.(not very good) A locking dif locks both wheels so they both spin all the time. Lets say you on a road. One rear wheel is on a patch of ice, The other wheel isnt. The wheel thats on the ice will spin and you wont go anywhere. A locking dif both wheels will spin so off you go.