The ballast resistor is designed to protect your distributor (the ignition points in particular). It sets the resistance in the ignition to 3.0 Ohms when combined with the resistance in the factory coil or 1.5 Ohm coils. In addition, it is needed to get an accurate tachometer reading when stock or 1.5 Ohm coils are used, as the tachometer takes its reading from after the ballast resistor.
Ignition Coil Note:
This ballast resistor should be used with stock ignition coils and 1.5 Ohm coil