commando steel shim head gaskets

Let me ask a stupid question. What’s the difference between using a steel head gasket and using no head gasket at all (besides the obvious difference in head cc measurements)? I always wondered about this. It’s pretty obvious that steel gaskets don’t compress much, if at all, so why are they used instead of nothing???

When I worked at an antique restoration shop, we got an original Alvis (British car) in to restore. It was a straight 6. Beside having to remove the cam from the rear into the passenger compartment, the motor used no head nor intake/exhaust gaskets. Can't remember about any other gaskets. I guess they depended on really close manufacturing tolerances.