"What is the intended purpose of a "Resistor" plug?"

A resistor plug, or resistor wires, damp the high voltage oscillations that would otherwise occur up 'n down the wire each time the plug fires. These oscillations radiate a tremendous amount of radio interference. The resistor wires -- or plugs -- dramatically reduce this source of radiated interference.

I don't know the answer to the question you haven't asked ("Well, if that's so, how come one of my Volvos specifies resistor plugs and the other does not?"). But an educated guess is that Volvo realized that resistor wires, as used on your earlier car, were found to be somewhat fragile and less reliable than the solid core wires used on your later car. By moving the resistors to the plugs, Volvo improved reliability.