Reason #1: Your electricity bill. Basically, anything that has a little light on it is drawing electricity when it's plugged in, whether you have it turned on or not. It really is a good idea to unplug computers and other electronics when you are going to bed, or going out for a few hours, or whatever. Of course, if unplugging your (insert device here) means you'd have to completely reprogram it when you plug it back in, don't put yourself through that kind of madness. Lowering your electricity bill won't do you any good if you've lost all your sanity.
Reason #2: The usual energy-saving stuff.
Do you unplug your electronics? Do you have another reason for doing it or not that I missed? How's your sanity?