Not such an unusual switch. It is probably a "sterile cockpit" light. These frequently illuminate a blue light overhead the cockpit door (externally to the flight deck) so that the cabin crew can determine whether "sterile cockpit" procedures are in force.
Sterile cockpit procedures are a method for minimising unecessary flight crew distractions during the critical phases of flight. They imply that the flight crew are observing a quiet/minimal distraction operation, with no extraneous chat. The external light indicates to the cabin crew that the flight crew are not to be disturbed with non-essential or non-emergency interuptions/distractions. Individual company procedures may require a "sterile cockpit" below an altitude of circa 15'000ft.
Either that or the switch ensures the death of whatever sperm (or ovaries) are left after all that increased cosmic radiation.