Pump shift lever

Why does the pump shift lever have the middle notch? Why is it not just on and off? This debate comes up once in a while curious to hear what people here say.

Let me rephrase the question and debate. I know the notch is there for manual pump. But if using it normally why does everyone in the fire service say to stop in the manual position and "pause" before going to pump? Manufacturer specifically says not to do it.

Sorry for the confusion