Who vacums when its pitch black???
Have you ever vacumed under the bed? In a closet? Under the table? Those are all grounds for breeding dust bunnies! You cannot always get all of them if you cannot see them!
I think they have lights to help the user see under furniture better or even to see the reflections of dirt on a surface. I know I use the lights on my vacuum to determine whether I really got all the dirt out from under tables chairs.
I, too, wonder at some of these things. Another one is why there is braille on the numbers at the bank drive through ATM. I do know that when the lightbulb is out on the vacuum, it is harder to see the dirt, even in daylight. I guess the lightbulb helps create shadows so you can see the dirt.
Lighted vacuums are like Heineken some commercials - reaches the underparts that other vacuums don't reach. Having a light on a vacuum is advantageous when you want to vacuum those dark corners where normal light can't shine. They scare away the shadows too. If you *do* try to vacuum in darkness with one of these, the lights only give you a few inches of light, so in my opinion, there's no real point in having them except for decoration.
It's so that oncoming vacuums can see you approaching.
