A question for the (gu)ages

I don't understand fog machines. What is their purpose to a live show? I mean, I suppose it makes the band seem more mysterious, but other than that I just don't see a point at all. Sure, if you're going to be at a club it makes those dancing look sexier and also mysterious, but then you bring them home and you have to force yourself to throw up. Then you peek out of the bathroom to take another look and you vomit again. Its never good. That's what I'm seeing with this. Nothing else. Fog machine equals ralphing.
Then again, if Velvet Revolver is doing it, then Hospital, MUST!