If you draw out your ignition system, you will see that it makes one big circuit. The coil, wiring, points, etc, all have some electrical reisstance. The ballast resistor is in series with the other components. If you decrease the resistance of the BR, then more current will flow through the other components. Too high of value, and not enough current. So too much current would over heat coil and burn out points. Too little may result in not having adequate voltage at the coil (and weak spark).