Here is an article from an Imperial site......
Purpose of Ballast Resistors from Brooks:
When the engine is running, you can get by fine with a lot less current at the points. That's the purpose of the ballast resistor. If the points were to receive full battery current/voltage at all times, they would only last a few hundred miles before needing replacement.
At the time of cranking (when the starter is operating), a separate wire gives the points the full 12 volts, bypassing the ballast resistor. When you release the key from "start" to "on", all the power to the points now has to flow thru the ballast resistor, preserving the points.
So that's its purpose....for those curious. And I know I'm kinda mixing volts/amps in the above....which isn't fair....but you get the idea.
Addition from Bob:
Let's be clear that there is no separate wire on the early models. The points get the current directly through the ballast. In order to provide the full 12 volts during a cold start, the ballast is also cold and has nil resistance. As the ballast heats up (some of them get VERY hot!) the resistance gradually climbs up to 1.5 th 1.9 ohms thus reducing current to the points.