We need the higher voltage to run the brains from somewhere. In these setups where we are supply the higher voltage to the receiver the red wire has to be in place so we can use it.

The mod into the switch wire needs the red wire so the receiver and servo can get its power from us.

It never hurts to check the 1S box even in 2S. In some setups that is what makes the low voltage cutoff work. In some cases like this switch wire mod the low voltage cutoff does not work no matter what you do.