Why wheels have a need for individual rpm

As long as a car goes straight all tires rotate at equal speeds (rpm).

Cars need to make turns. In a turn the inside tires need to rotate less than the outside wheels.

A solution is needed.

The differential

A differential is able to split the rotations delivered by the drive shaft either equally or proportional to left and right as needed.

The differential is a mechanical device that reacts to resistance. The inside tire, by following a smaller circle in a turn, puts up resistance. The differential reacts to this resistance and allows it to rotate less. At the same time the outside tire is forced to rotate faster by the same rate the inside wheel rotates less.

Remember that the differential reacts to resistance - that behavior creates an unwanted effect that needs control.