Ya missed the point, Yeelep...
Think about this for a moment....in an NG you arrive normally at the gate, external power is plugged in and the engines are shut down!
All normal checklists are completed and you depart the airplane, looking forward to going home for the day!
The airplane is scheduled for another couple of legs in a couple of hours...
Some time after you leave the airplane, the external power fails or gets accidentally pulled from the receptacle...
With the AC power failure inside the airplane, the Standby Power system immediately kicks in and powers all those good things you expect it to do.
Now it sits there at the gate with all this stuff powered (won't be everything that's normally powered though, since the #1 IRS, engine R IGN and the lighting will all be switched off at this time), until the next crew turns up to start their preflight prep.
By this time, depending whether this airplane has a single or dual battery system, the standby system will be almost or completely dead.
This will mean one/both batteries will need to be changed, since there's not enough time to get them fully recharged before the upcoming flight.
Have fun....I'd rather simply select standby power to OFF between flights.