The resistor acts some what like a resistor plug. I'm not really sure of why things work as they do, but doing range check you can see what the result is of adding resistance and/or shielding.
The ancient standby has always been a 1w 10K resistor in the lead. Still works good today.
There are two parts of noise one is the spark itself, a spark has an infinate number of harmonics. The other is these ignitions don't have a smooth steady current draw. They take a big gulp of power just after the plug fires to recharge the HV cap. This puts a noise equal to RPM on the battery leads, switch and the battery its self.
Modern raidos are so good that we can get away with many sins in our installlations