Why a Resistor?

[Henry Ford IV]

I know Henry Ford is right, plus he wouldn't have put it in there if it was not needed, but why is there a resistor in the 6 volt system, going to the coil and points? In a 12 volt there is a resistor that knocks it down to 8 volts or so, so the points don't burn out. We've got the same basic points, at 6 volts they should be OK if they are OK at 8, so why knock the voltage down to 4.5?