I just got a pair of computer speakers that have a car cigarette lighter plug added in near where the power input is. Turns out this is completely clever. Why? This standard 12 volt connector (the cigarette lighter plug) is wired into the 12v side of the transformer inside the speakers. This transformer takes 120v from the wall and converts (transforms) it to the twelve-volt level the speakers run on. So, if you have your speakers plugged in to the wall you can just tap the 12 volts and use it to power any other gadget you want that runs on 12 volts.
But if you’re in the desert, and maybe all you have is a (12 volt) car battery — just connect the 12v battery to the 12v plug, and voila — your speakers are running off the battery. In the first case the plug is a source of electricity, and in the other it’s the input. Totally genius.