If you want to see why it's needed, hook up a pressure gauge to the mainline pressure, start the car and move the kickdown lever while watching the gauge. The more pressure the tighter the clutches are held. The transmission shifts between two pressures throttle pressure and governor pressure. the more throttle pressure, the more mph you need to develop the governor pressure to make the shift valve move. throttle pressure is also use to provide load to the valve body, as throttle pressure increases so does the mainline pressure. Higher the load the more line pressure you need.