[Aussie Mike]

The aluminum hat section is a significant weight saving over a conventional rotor. Also if you machine your own hats it opens up a lot of possibilities for different offsets.

Also a lot of 2 piece rotors are also set up as floating. The rotor can move side to side a little independently of the hub. This means you don't feel so much brake shudder through the pedal as the rotors warp under extreme conditions. Its usually a race track only thing since the floating mounts are another wear point you really don't need on a road car plus they can be noisy.

Cheers