It's to force the train to be on tap X. When doing it via the 'other tap' (0-30), you give only the max tap that you wan't to give and the train max itself which is a lot of times lower than what you have put. So the up/hold/down is to force to be on that tap and the other thing is giving the max and the train limits itself.
^ This. The up/hold/down is there for malfunctions. Should the Nachlaufsteuerung (it's the German term for the system that lets the loco dial up the taps according to your tap on the wheel) fail, you use these to manually control the loco and make it to the next depot without needing a rescue.
BTW why is it that the tap changer always resets to 0 once you try to change maximum? E.g. when you selected maximum 12 and then want to change to 15, power goes down to 0. When you then select 0 and increase from 0 to 15, the tap works again. I know that you have to select 0 before applying the brakes, but the manual did not say anything about going to 0 before changing the maximum tap.
