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.