Arguably, the same. You can watch videos in what amounts to a very large screen, with ambiance added to the backdrop, anywhere. Watching videos on a plane or train will get a massive boost.
Given that an iPad Pro with max specs is about 2/3 of the announced price of the goggles, I think the goggles are in fact at an advantage.
Not sure what your reasons are, but on a 30min-1h trip on the London Tube, I can see the appeal and not many reasons to not use it. Keep the "real world" somewhat visible for awareness and enjoy. Not much different from taking my laptop out and coding on it.
Well, the trivial answer given your premise that the iPad's existence is justified because some people sometimes might prefer it over other screens is that some people might similarly sometimes prefer the Vision [Pro].
I think it's a lot more compelling than that, personally, but you didn't set the bar very high.
Give me one thing that those goggles let me do or do better right now.