Inline ballast resistor or resistor wire(GM) was used to extend the life of older coil designs.
If you have one inline then there will also be a second pathway to feed the coil the full 12volts but only when starter motor is on.
Newer MSD Coil would not need the resistor inline, they have better cooling.
When engine is running electrical system should have 13.5 to 14.9volts, the resistor would cut that voltage down to about 8-9volts, which is plenty of voltage for running engine spark, and the lower voltage kept coil cooler, so it lasted longer.
But when starting the engine, the starter motor draws alot of amps, so battery voltage drops to about 10volts, if power to coil was still flowing only through the resistor the coil would only be getting 4 or 5volts, very very weak spark to start cold engine.
So a second pathway is needed to send the coil that 10volts during cranking, to get hot spark.
This was done either thru the ignition switch, key in START position, or via the Starter Relay(solenoid) on the inner fender, the "i" post on the older 4 post starter relays.
Leaving the resistor in won't hurt anything but with newer coils it isn't needed
Common issue on the older systems were:
Engine starts but dies as soon as I release key from START position, resistor is bad or connections corroded
Engine cranks but doesn't start, but seems to want to start/fires once when I release the key from START position, resistor pathway is good but 12volt pathway(key in START position) is disconnected or corroded connections.
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