If you're running multiple lights/rooms, it pretty much evens out your power consumption so you don't get tell-tale 'surges' of energy use. And it is supposed to make the overall power consumption slightly lower.

Something I used to do before I had my wiring updated, and this is a good idea for fire-safety, was to have my lights on separate timers so that one would come on about a half hour before the other and there is never a time when 2 or more HIDs are charging up to ignite simultaneously.