Question

What is the difference between the lighting circuit and power circuit?

Answer 1

Power circuit carries high power and costly devices. If there is some unwanted power signal(noise) in the wire it can damage the device. To reduce this effect earth is necessary.

The lighting circuit carries low power(current). The wire diameter is smaller and as a consequence, cheaper. Because the fittings in a light circuit are usually out of reach, an earth wire is often not used. Again a saving in the cost of an extra wire.