In almost all automotive wiring the car body is the other or ground wire. Steel is a rotten conductor of electricity, but it is free, so the factory uses it frequently every chance it can. That is why there is only one wire powering most devices. The prong is a power tap for anything that needs a 30 Amp circuit that doesn't go through the ignition switch. Since your alternator only outputs 37 Amps wide open you can see where that would be a health battery drain if you where to hook that prong up to the ground.
It is a great source of power for a power bus of your own making to power things like an aftermarket radio that has an internal clock and as such has to have power all the time to remember your favorite push button station settings or the aforementioned digital clock.