This is occasionally used as a synonym for 'ghost load' a lantern
connected in parallel with a stage-practical to ensure the dimmer has
enough load on it.

More usually it's a light on a pole left on-stage while nobody is about.

The practical advantages are that the last people out & first people in
won't fall in the orchestra pit in the dark.

The historical reasons for it's existence (gaslights acting as pressure
relief valves, keeping the ghosts away, keeping the ghosts happy,
ensuring the theatre never 'goes dark' etc.) are occasionally discussed
to death on r.a.t.s. - do a dejanews search if you're interested.