Why Low Voltage used for Outdoor Lighting?
The reason low voltage (12 volts) is used for outdoor lighting is that, like batteries, it will not produce an electrical shock if contacted. Thus, the codes, precautions, hardware and hassles attributed to 120-volt use need no longer apply.
As for light, a 12 volt bulb is indistinguishable from its 120 volt counterpart. And efficiency? The same goes for the bulb itself. However, getting energy to a bulb is another story altogether.
The percentage of voltage loss [in the hook-up wire] quadruples for every system operating at half the voltage. Comparing 12 to 120v systems, the same wire used in the 12 volt system will reach a 10% voltage loss with just 1/100 the wire length (now you know why 220 volts is used within a home for powering an electric range).