A lot depends on what you are doing.
For 3D planes, the high rate is for extreme control surface movement and low is what is needed to fly the plane in normal flight.

If you are doing aerobatics, high rate may be what is needed to perform the maneuvers and low rate is what is neded for spins and snaps.

You can also set it up to simulate flight modes on two or three controls surfaces. How you use your rates switch is up to you.