> it adds very little on top of what I was already able to do by saying "this is PR 1 out of 7, this is PR2 out of 7 and build on top of the branch that I used for PR1/7, etc".

That's literally all it's supposed to do. Make that dev flow less awful so you don't have to say "by the way this PR depends on #123, I will change the target branch when that is merged" and nonsense like that.