Staging is immensely useful in more than one case.
Case one: WiP on a branch, code is pretty stable, but I want to do some experiments which will likely be deleted. I stage everything and then make (unstaged) changes which I can then undo with two keystrokes.
Case two: I'm reviewing a complex PR, so I first merge it with --no-commit, then unstage everything and then stage chunks (or even individual lines) I have already reviewed.
Case three: I was coding in the state of flow and now I have a lot of changes I want to commit, but I want to separate them into several atomic commits. I stage the first batch of changes, commit, then stage another, commit, etc.
There are probably more, but these three alone are worth having the staging area.
Case one: you don't need staging. You can stash, or just commit and checkout HEAD^
Case two: you don't need staging. You can stash, and then unstash incrementally (I would be shocked if git doesn't have the equivalent of `hg unshelve --interactive` and its keyboard friendly TUI)
Case three: you don't need staging. You can just edit/amend the series (rebase --interactive I believe you guys call that).
That is to say, all that you want to put in your stash, you could commit directly to the DAG, and edit to your convenience, with the regular history-rewriting tools already at your disposal. And the off-DAG stuff can be handled by stash (and even there, a normal commit that you would rebase to its destination would perfectly do).
And what I described is incidentally what https://github.com/jj-vcs/jj does, which can be pretty-well described as "taking the best of all major VCSes"
Case one: WiP on a branch, code is pretty stable, but I want to do some experiments which will likely be deleted. I stage everything and then make (unstaged) changes which I can then undo with two keystrokes.
Case two: I'm reviewing a complex PR, so I first merge it with --no-commit, then unstage everything and then stage chunks (or even individual lines) I have already reviewed.
Case three: I was coding in the state of flow and now I have a lot of changes I want to commit, but I want to separate them into several atomic commits. I stage the first batch of changes, commit, then stage another, commit, etc.
There are probably more, but these three alone are worth having the staging area.