What's so special about the flat enamel hanging grill?

You are grilling and frying at the same time; there’s plenty of enamel coated metal in it. This means you can start cooking sooner and don't have to wait for the fire to burn down to charcoal as with a conventional barbeque. The chain allows you to alter the height above the fire/embers, and you can swing the grill around to vary the heat.

What is the little handle at the top of the tripod for?

The tripod comes with a fixed chain and a twiddle handle on top. This can be used for turning the grill over the fire for an even cook.

Is it better to have the grill attached to the t ripod legs or, as with your set, hanging from a central chain?

The enamel grill hanging from the central chain is more flexible. The chain does not get hot and you can move the grill up and down the chain without a glove. You can also spin/turn the grill over the fire, using the twiddle handle if your fire has hot spots.