because you can leave the aux switch on at all times. When you shut the truck off, the aux switch loses power, so it can remain flipped on at all times. So with this mod, your parking lights are on at all times when running, and your headlight knob can be in auto mode.
Without this mod, every time you get in the truck you have to turn on parking lights, every time you get out you have to turn off parking lights, and every time it gets dark you have to turn on your headlights, rather than use auto mode.
I'd prefer this mod with tapping into a switched 12v source instead of using an aux switch. It should work the same, just free up an aux switch.