During some starting situations, the load of the starter motor will drop the battery voltage to about 9.5 volts. Using a coil that needs 12V to fire well results in a weak spark during a cold start when you need a good one. By using a coil that works on 9 volts, and dropping the voltage from 12 to 9 during normal running, then bypassing the resistor during starting, you get a good spark all the time.
Jerry
