The car's transmission works to transmit the right amount of power from your engine to your wheels to travel at the speed you desire. The importance of the transmission's changes in power and speed, is how it reduces the load on the engine, while increasing the vehicle's speed and fuel economy. The health of your transmission allows you to drive at maximum efficiency.
If you avoid getting a Transmission Fluid Change, it will cause excessive friction which can damage your internal clutches that shift the gears.