I do not know if it enters the definition but it's my first step for trying "dangerous" stuff, like debugging a macOS kernel extension, or installing an exotic

I do not know if it enters the definition but it's my first step for trying "dangerous" stuff, like debugging a macOS kernel extension, or installing an exotic OS. And of course for learning. For example you can learn a lot about operating systems if you can just run any older version at your will. Also, you are obviously speaking of virtualization of a given hardware platform but a lot of stuff is modeled as a "virtual machine" in the generalized sense - e.g. the pickle Python format.