We all already own an outdoor light. It's called "the sun." But let's set that aside for a moment, okay?
There are places around your house where the moon doesn't reflect, or maybe the sun doesn't brighten enough. There are corners where the shadows land just so, where a burglar or bear or three raccoons in a trench coat could secret themselves. And in those dark places, you don't want a series of mirrors that can slowly be aligned across the space of an hour to draw the light across the sky. You want a light you can just turn on, instantly, with no hesitation, so you can tell if that shadow is a prowling madman or just a stack of leaves. And THAT'S why you need, as it were, a second sun.