Think of it as similar to having "jumper" cables between the chassis and house batteries. If the house batteries went dead and you can't start the generator, holding the aux start button will connect the house batteries to the chassis battery which would allow you to start the generator. The opposite is also true, if the chassis battery was low or dead, then holding the aux start switch in would allow the chassis battery to pull power from the house batteries.
On mine, and I think most still today, if the RV engine is running, then the aux start relay is engaged (the same as if you held the aux start switch in) which also allows the house batteries to obtain a charge from the engine battery and alternator.
In summary, if you ever find that either the house battery(s) or chassis battery(s) are dead and too low to start the generator or the RV engine, then the aux start button can and will be your friend. If you do encounter this (dead battery), it would be best to start whichever engine you can (generator or sprinter engine) prior to using the aux start switch. ~CA
BTW, Welcome to the Jayco Owners Forums (JOF).