It saves a bunch of wire.
It's a long distance for the ignition run wire if it went to the trunk and back to the engine compartment. A larger diameter wire would be needed to be sure there was the same or less resistance than factory.
In theory you could run just a field wire to the kill switch. Resistance would still effect alternator performance but not voltage regulation. However given a choice, I would want a kill switch that disconnected the ignition along with the power sources.
