What's the purpose of adding a parallel impedance to a relay circuit?
It isn't purely resistive - it's mainly reactive due to the 100nF capacitor. At (say) 50Hz, 100nF will have an impedance (reactance) of nearly 32kohms.

The resistor is there to absorb any spikes that may otherwise cause damage to the relay contact. It's called a snubber circuit and is often seen in parallel with contacts too.

A lightbulb will be purely resistive but there could be a reason in the designer's mind that maybe a motor could be connected and this could cause inductive kick-back voltages of thousands of volts.