As long as a car goes straight all wheels rotate at equal speeds (rpm) and both axles receive equal rpm from the drive shafts.
Cars need to make turns. In a turn the front axle needs more rpm than the rear axle:
In a turn each wheel follows a different circle. The sum of both rear wheel rpm is 100 - equal to the drive shaft rpm. The sum of both front wheel rpm equals 104 - more than the 100 rpm the front drive shaft delivers. The tension created leads to unsafe driving and destructive stress.
A solution is needed
The center differential
A differential is able to split input rotations delivered by the transmission either equally or proportional to front and rear axles as needed.
A = center differential
The (center) differential is a mechanical device that reacts to resistance. The rear axle, by following a smaller circle, puts up resistance. The differential reacts to this resistance and allows it to rotate less. At the same time the front axle is allowed to rotate faster by the same rate the rear axle rotates less.