HotRod is correct.
There's a detail he left out, however. Without a ballast resistor, over time, metal will be transferred from the outside contact of the points to the inside contact (due to the heat caused by excess amperage as well as the opposing charges of the two contact surfaces. You actually end up with a cone shaped buildup on the inner point contact and a corresponding hole on the outer point contact.
Naturally, this changes the point gap, because the cone is closer to the edges of the hole than the flat surface surrounding it are to each other(which is what the points were gapped to). That can dramatically affect performance and even prevent starting.
The reason the ballast resistor was bypassed during cranking was to provide a higher voltage spark to the plugs, making it easier to "get the fire going". The combination of no heat and cold fuel required a hotter spark to ignite.
Once the motor is running, the plugs don't require as much voltage to ignite the fuel-air mixture because the environment is a high enough temperature to almost ignite the fuel without a spark.
The reason diesel engines don't use spark plugs is because the compression is so high that it generates sufficient heat to ignite the fuel without an ignition system. Of course, back in the old days they used to use "glow plugs" to help warm up the fuel air mixture until the motor was running. Compression was lower back then, and the engine needed a little help to get going.
