If they're moving the selector to ON on the ground then it expedites the APU start when START is selected. Therefore, the APU can be started later hence it saves a few kilos of fuel.
I think you'll find it's more important to ensure a good APU start, rather than have to go to the gate then wait for a ground cart to plug in so you have power before you shut the last engine down .... while it's burning a good 800kg/hr.