Anything metal in the pool must be bonded to ground to prevent the possibility of a voltage potential difference between any two items.

So is the ground to prevent electrolysis

Frankly, if you don't understand what is going on, you shouldn't be messing with electricity in a tank of water that you are immersing your friends and family in. We lose a few people each year to faulty pool lights.

As HU points out, every exposed metal in the pool need to be bonded appropriately to avoid the possibility that it doesn't become dangerously energized. The fact that the bulb is only 12V doesn't mean that the circuit is immune to a higher voltage in a fault.

That light ground needs to be connected by #8 solid to the rest of your pool bonding system.

I fully understand what's going on and that the 6ga ground wire currently in the conduit is bonding for the niche. I am confused why my 12v light fixture appears to be the only model with a ground. The circuit should essentially be immune to higher voltages. There's redundancies built into the transformer and the GFCI that powers that transformer should trip should an issue occur before the transformer.

This is why you are in trouble. I explained below.

If the thing has a ground YOU MUST CONNECT IT. The manufacturer put it there for a reason.