Switching to ON from OFF opens the APU door and the fuel valve, arms the APU bleed isolation valve and activates the fuel pump (DC or AC). If you put the switch to START then release it to ON, all the preceding will happen and the APU starter will engage when the door is fully open.
On the Classic you have to wait until the APU door is open (blue in-transit light out) before selecting START. Selecting START before then does nothing.
Maybe there's an operational benefit in opening the door, thus ventilating the APU (which will be cold soaked), followed by a rapid start when approaching the stand. Or it might be old habits taking time to die out.