If you get PsTools, and do this:
psexec -i -s CMD.exe
you’ll get another CMD box, where the username is SYSTEM, that is to say, the hostname of the machine with a dollar sign on the right end. If the machine is on a domain, it is DOMAIN\hostname$, have not tested it on a non-domain machine yet.
One software installer recently, required that the folder containing its installer package be TAKEOWNed and ICACLSed, as that system user, before it would run to completion, it must have some odd permissions bug in it.
There are likely to be quite a few circumstances in which this special CMD can be useful.