because wires can get really hot and start a fire
L5-30R is specifically a 125Vac, 2 pole, 3 wire locking receptacle. It would be wired the same as a standard 5-15(aka standard 3 prong), except a larger wire guage is typically used back to the service panel. The receptacle itself will be color coded black, white/nuetral, and always green/ground.
They protect the downstream components from spikes in the eletrical system.
With a resistor. You step down from 12 volts to 2 volts (yes, its 2 volts, not 3) with a resistor. A 2 ma LED would require 10 volts across 2 ma, or 5K Ohms. (If you want 3 volts, then you need 9 volts across 2 ma, or 4.5K Ohms.)