Almost everyone commenting is not an actual photographer. Maybe they use their iPhone as a camera but I bet most people only do set-up shots where they tell the subject where to stand and they say "3, 2, 1 Snap". The bare cell phone is ok for that kind of shot.
But what if you photograph the real world, not the one you set up? Then you are looking at the subject and can not possibly take eyes off of it to muck with an iPhone menu system or on-screen camera controls. With a real camera, I have controls I can use without the need to look at them. This device would allow me to interact with the subject and not the touch screen.
I have a Leica M6, and a Canon RP with various lenses. And I can tell that you have no real idea what you're talking about.