The primary winding in the ignition coil has a low resistance. If the ignition were to be left on with the points closed, considerable current will be drawn through the system. The ignition switch, the coil itself, and the points. Even at low engine speeds this can be a problem because at lower RPM, the points are closed for a longer duration, than say, highway driving.
The resistor wire increases the resistance of the circuit reducing the current. The additional wire that goes to your starter, doesn't technically "go to" your starter, but instead "comes from" your starter. The starter solenoid has an extra terminal that deliver power from the solenoid to the coil, bypassing the resistor wire altogether.
This gives the coil an extra "boost" of current to aid in starting. This has the added effect of compensating for lower available voltage during cranking. Because starting should only last a few seconds, the extra current doesn't present any danger to the system.
Enjoy!
Regards,
Mike