I think it was a very valid question. I was, and still am, wondering the same thing
I guess you could question the specific example, but to me the question sounded like "why would you do rebases".
Like, for various reasons?.
The point was that it's way easier and way safer to do rebases in jj, and it straight up allows to do complex stuff like the mentioned example easily and inderstandably, idk
Also this is just an example, there are a lot of very complex (from the point of git) things that are trivial like that in jj.
Although if you're asking "why would we do that" then I don't even know what to tell you lol