It normally depends on whether it is a plastic housing or a metal housing. IF metal, the ground wire is normally connected to the metal casing. This way, if one of the power wires shorts to the metal casing, the electricity will flow to ground and trip the breaker. If one of the power wires shorts to the metal casing without a ground wire protection, someone could touch the metal casing and get shocked or electrocuted, because the electricity flows to ground through their body.
A plastic casing does not conduct electricity. That is why some plastic appliances have 2 pronged plugs instead of three prong plugs. There is no ground wire from the plastic casing to ground. The third plug goes to ground.
The metal casing light fixture will work and light up without the ground. As long as there is no short to the casing, you don't have the danger of shock. However, if the lamp shorts to the casing, you have a dangerous situation.
Yes. General questions require general answers.
if the fitting has metal parts and is not double insulated
