The purpose of the relay that comes with the True Isolator is to provide a "switched ground", so the voltmeter won't be on all the time on those installations not switching the AUX battery. If the AUX battery is switched (by key switch or my manual switch) there would be no need to switch ground on the voltmeter to turn it off when not in use.
Some may want their voltmeter on all the time. That way they can just glance at it to check voltage. The voltmeter doesn't draw a lot of current, less than 10 or 12 mills IIRC. However, it would tend to drain the battery on a machine that was operated infrequently.