With rotary switch you switches from one value to another. If the rotary switch used have 4 positions, then 4 inputs values are available corresponding to 0V, 1.1V, 2.2V, 3.3V.
With potentiometers, if the current position of the potentiometer is on the threshold (virtually created by rounding the input value) between a value and the next, then the output can oscillate. You don't have this problem with rotary switch.