On the early Airhead points type models, spark plug caps contained a resistor of 1000 or 1200 ohms. Using a resistor inside the caps is a better method than using highly variable (between manufacturer’s especially) “resistor” type spark plugs, in which some of those resistors were actually coils.  The resistance caps reduced Radio Frequency Interference, and helped form the proper type of electrical spark itself.   Later Airhead points models and all electronic ignition models used 5000 ohm caps for even lower RFI, with added safety benefit for the 1981+ electronics ignition…described later here.

The spark plug cap resistance has more than one purpose:
It reduces spark plug tip and ground electrode erosion, and therefore greatly reduces any gap change over the life of the spark plug, and does this by reducing the electrical current flow. It does NOT reduce the applied voltage. Reduces some types of radio interference. Works in a complicated electrical way with the coil to produce a quality spark at the spark plug for good fuel mixture igniting.   There are some other beneficial and rather complicated effects as well. Note that if the resistance is too large a value, then the EFFECTIVE spark ENERGY (combined voltage, current, and with SOME time effect) will be lessened too much, and, this means that using resistor spark plugs WITH resistor caps is a BAD IDEA.