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.