Holy Zombie threads.
Clearly it's so you can get stuff from the trunk while driving so you can practice juggling while driving!
Of course it's for while you're outside the car. I wired mine to turn on when the trunk is open. The trunk area is pretty dark, even with the back open. And those tall seats cast a huge shadow into the trunk, even if the map light is on. Try finding a black tool kit, lying on a dark blanket, at night, in a smart trunk. It's not easy without some light in there, and the map light just doesn't cut it with the seats in the way.