From the ignition switch, power goes first to the ballast resistor - then to the coil primary terminal (+). When the engine is running, voltage to the coil is reduced to about 7 volts. 

When the ignition switch is turned to "start", power goes to the solenoid (through a neutral safety switch) - which engages the starter motor - and a "jumper" is also routed from the fourth terminal post to the coil side of the resistor. The heavy starting load on the battery makes the voltage "sag" - so the jumper delivers something close to running voltage (7.0 volts) at the coil primary.

The original coil turns ratio and inductance were designed to deliver what was acceptable spark energy for the date and time.

Later Ford ignitions replaced the ballast resistor shown in the circuit with a "resistance wire" from the ignition switch. This accomplishes the same thing (cuts voltage at the coil in "run") with less localized heating - and it was cheaper to make. The starting jumper is a good feature and still remained even when the ignition went to electronics (Duraspark II). When reconfiguring engine wiring this jumper should be retained.

When I've checked the Ford coils for resistance - the mid 50's were at about 3.0 ohms and the 70's electronic variety were 2.7-2.5 ohms. Both types were oil filled to cool the windings. The resistance wires are also pretty much comparable to the old ballast resistors. The combination of the two (coils and resistors) reduces the current flow in the system to a value that provides reliable service life - for instance, running an oil filled coil with no resistor can "cook" the windings.

If you change the distributor to a later version - and you stick with the Ford oil filled coil - use a ballast resistor. There are "hot rod" epoxy filled coils that will tolerate more heat in the windings - and they usually have directions that will tell you if a ballast resistor is needed. The points will tolerate this with reasonable service life as long as you have a good condensor.