The analogy wasn't that great. Booting a machine has nothing to do with typical uses of Emacs. There are many applications which are long running and are working on multiple documents - it's just another mode of working with an application.
One can use GNU Emacs just fine as an editor which starts and quits on a single file. 'emacs -q' starts in a few milliseconds on my Mac. I use single file edits with Emacs on the terminal all the time. Though I usually also have a long running instance of Emacs - but then it is used as a Common Lisp IDE, not just as an editor.
Then one can setup an Emacs editor server.
Or start the Editor and leave it running. This is no different from, say, a CAD program or any typical application on the Mac, where usually a single instance of an application keeps running, editing multiple documents.