Some people will use them for things like flaps, so rather than having a certain position, they can set it on an analog rate; they can slowly dole out the level. Or, it might be used for a camera position for FPV, like on a camera drone; maybe you move it to a certain angle with one knob, and are rotating it to another angle with another knob.
I have one set for the radio volume, and depending on the model setup I have used them to control the amount of rudder/aileron mixing, the servo rates (rather than just high-low having continuously variable), variable expo, the amount of differential thrust mixed into the rudder control, and gyro self-leveling strength so far.
I don't see why not; you'd simply be assigning it to a channel from the receiver, say, AUX1 or AUX2 or something similar. You'd then just have to assign it to the appropriate knob.