OK, when electricity is being used in a metal container or metal hood, there is a possibility electricity can "leak" onto the metal. It could be cause by broken wire insulation or moisture. Lets say the hot wire (Black) insulation wears due to vibration or the metal cuts into it. That makes the metal energized but the circuit is not complete as it has no way to ground. Your lights work so whats the problem? If you touch the energized metal you can become the ground path. Electricity will flow from the metal THROUGH YOU to ground and you will get a shock up to the full current of the circuit.
If the metal was grounded when that fault happens, it would cause the breaker to pop as it is a short. If it is "leaking" electricity such as from dampness, the ground will dissipate it harmlessly and no shock.
That being said, a ground fixture is not grounded unless the other end of the wire is actually connected to ground.
Since indoor growing involves water and electricity,
YOU WANT THINGS GROUNDED.
I lived in an apartment that did not have 3 hole grounded outlets, only the hot and neutral. It also had radiator steam heat. I put in an air conditioner in the window during the summer and broke off the ground part of the plug to make it fit the outlet...
If I touched the air conditioner and radiator at the same time, I got one hell of a shock. It was 90 volts on my voltage meter from metal case to radiator. I connected a wire from the AC to the radiator and no more shocks.
GROUNDED IS GOOD! GROUNDING IS YOUR FRIEND!