Switches
Switches have a lower DC voltage rating because when arcing starts in DC you can't stop it other than by separating the conductors. In an AC circuit, the current drops to zero twice each cycle, so the power is automatically cut off, stopping the arc, hence the higher AC voltage rating for a given switch.
An incandescent lamp will draw less current with a lower voltage, but will also put out less light and the color will shift toward the red end of the spectrum. An incandescent lamp operated 5% above its rated voltage will have its life expectancy cut in half. One that's operated 5% below its rated voltage will have its life expectancy increased at least four-fold. Overvoltage is a killer for incandescent lamps.
Dean