Police advise mosdos to keep them on for securty purposes. Vandalism is much much less likely if there are lights on
Sooooooooo
1- it deters potential vandals
2- If the rooms are visible from outside the building, one can see IN only if it is light inside
3- Some bulbs are equipped to be cheaper ot run continuously instead of turing off and then needing 15-30 minutes to warm and illuminate when turned on.