And now, for the CORRECT answer. Ballasts change resistance with current as they heat and cool, and thus provide a sort of buffer for the coil. Coil current will be different at higher and lower RPM.
But the REAL nitty gritty reason is "easier starting."
A normal system runs close to 14V, and Mopar coils used to run anywhere from 8-10V AT THE COIL at the bottom end of the ballast.
When you crank the engine for start, the switch bypasses the resistor, so the coil STILL GETS about 10V--so a nice hot spark for starting.
This is something the ol' 6V cars never had.