It's for current limiting. If the coil is allowed to pull full current for extended periods of time it will burn up. It's a double because during cranking power is fed through one side of the resistor, which is actually just a bypass so the coil gets full voltage while cranking. When key is in the run position power is fed through the other side of the ballast resistor, reducing voltage and current. Aftermarket ignition systems like MSD have a current limiting circuit built in so you can feed your coil straight +12v.