How is the Cessna 172S G1000 Electrical System powered?
It would appear that there is a standby battery controller PCB.  It's important to note that M BUS VOLTS are read off the crossfeed bus as there is no main bus.  The label is a bit misleading in my opinion.  It's been awhile since I have flown a G1000 C172 but if I remember correctly you should not see 1.5volts on the M BUS VOLTS but zero.

The checklist is verifying the PCB box doesn't have a fault that will allow the standby battery to power more than just the essential bus.  Unfortunately, the POH is silent as to why this check is done.