I pulled this from NGK's web site and I hope it helps:
2. What is RFI?
Radio Frequency Interference (RFI) or electrical
"noise" is caused by ignition systems, alternators,
wiper motors, etc., but the ignition system causes the
strongest "noises" and they include a wide range of
frequencies. This noise can cause interference with
the operation of radios, electronic components, fuel
injection systems, depth finders, and other sensitive
electronic equipment. Resistor plugs, wires and
spark plug caps are used as a RFI suppression
system to reduce this noise.