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???
The embossed steel gaskets do seal. The "no gasket" theory won't work. Too much pressure (compression) and a mix of that and coolant and oil.