To continue with the heavy industry examples, many run 24/7, not because of direct profit motives, but because of the massive cost associated with letting the process go cold. I've done work at a glass recycling plant where the said point blank: if the furnace ever goes cold, they will just close the plant entirely, since it would cost too much to clean it out and get it going again. Not all processed lend themselves to being turned on and off again constantly.