It is just for the harness. Using a Mac Mini gives you direct access to Apple services, but also means you can use AppleScript / Apple Events for automation. Being able to run a real (as in not-headless) browser unlocks a bunch of things which otherwise be blocked.
I didn’t create this, but on the surface it looks like a reasonable median between “Superpowers”[1] and “Gas Town”[2]. Posting as I don’t see any discussion on it here yet.