Lites on the ground is a good indicator that "Power" is on, on the aircraft. If it is unattended the company engineers will probably investigate and shut it now if it is not due out for a few hours or whatever.
It also Saves the battery!!
As at night or during the day, if lites are still on although the GPU/APU etc are shut down; it is an indication of Power still on, albeit only the battery.
I think that makes sense???