Reuseable, as in great for a combination that might pop the heads off and on rather regularly. (top fuel).
The combustion pressure seal is greater when looking at the stainless steel lock wire, accompanied by a reciever grove in the head. You can lock in a bunch of pressure, where a conventional gasket and fire ring combination would/could be prone to failure. (again think of top alcohol-top fuel cars where cylinder combustion pressures are in excess of what most street cars see in detonation)
Detonation failure resistant, and or allow a builder to use a greater amount of torque to compress head to engine. Oversized bolts and or 18 head bolt combinations..... better more even clamp load than an MLS type gasket where the graphite can compress unevenly with the uneven loads of fastner space arrangement.
And as David has pointed out, hundreds of gasket thickness combinations to allow finite adjustment of combination.
Sealing water is a pain in the behind, most tend to leak water no matter the system used. On dry block engines like top fuel and or top alchol, no big deal, no water to worry about. Street engine and or saturday night racer...... a cylinder full of water can be a very big deal, and or a puddle under the car, rather disconcerting.
A very fine finish, similar to the recomeded deck and head finish for the COMETICS works well, espeacially when the gaskets have several layers of copper coat on them. Another sealing trick, I've seen spring car engines done with rubber O rings in block and head around the water passages. And lastly, I've used a variant of the rubber O rings, with a thin smear of RTV, allowed to set nearly completly before bolting the thing up. If all else fails, then use a ceramic coolant sealer in the system after assembly. Will seal till next dissassembly.
Some people use head gaskets as investment protection for other parts. Expecting the head gasket to blow when the tune is off and not damage piston, rod, block, etc.. The copper is going to hold that force in, and if its to great for the mechanicals, it will crush em.
As for fire channels through the aluminum, yup. Been there. The copper head gasket survived, although it was somewhat burnt. Had a nice 1/8" fire channel from combustion chamber to exhaust side of head.. Good excuse to mill the heads and increase combustion efficency.