What does the Git staging area actually do, and why not commit straight from the working directory?

Git keeps three versions of your project in view: the working tree (the files on disk), the index or staging area, and the last commit (HEAD). git add file copies the current content of the file into the index. git commit creates the new commit from the index, not from the working tree. The index is simply the draft of your next commit.

git status reports two comparisons. Changes to be committed is the index against HEAD. Changes not staged for commit is the working tree against the index. Likewise git diff shows working tree against index, and git diff --staged shows index against HEAD, which is exactly what the commit will contain.

That explains the file that appears twice. Staging takes a snapshot of the file at that moment. If you edit the file again after git add, the index still holds the earlier version and the working tree holds the newer one, so the file is both staged (old edit) and modified (new edit). Committing now would record the earlier version only. Run git add again to include the later edit.

To take something back out of the index without touching your file, use git restore --staged file.

The point of the extra step is that a working tree rarely contains exactly one logical change. You fix a bug, and along the way you also rename a variable, reformat a file and leave a debug print in. With the index you can commit those separately. git add -p walks through the changes hunk by hunk and asks which to stage, so even two unrelated edits in the same file can go into different commits, and the debug print into none.

Focused commits pay off later: a revert removes one change and nothing else, review is easier, and git bisect can point at a small commit instead of a day's mixed work.

git commit -a stages every modified and deleted tracked file and commits in one step. It is fine when the whole working tree is one change. Two things to know: it does not include new, untracked files, which still need git add, and it bypasses the review that staging gives you, so make a habit of reading git diff first.

One risk of partial staging is that you commit a combination of changes that never existed on disk and so was never built or tested. To test exactly what is staged, git stash --keep-index sets the unstaged changes aside so the working tree matches the index; build and test, then restore them with git stash pop. Running the test suite in CI on every pushed commit is the backstop for the same problem.

The index has a second job during merges. When a merge hits a conflict, Git records the conflicting versions of the file in the index and writes conflict markers into the working file. After you have edited the file to the result you want, git add is how you tell Git the conflict is resolved, and git status lists which files are still unmerged.

Seen that way, the staging area is not an extra hurdle before committing. It is the place where a commit, including a merge commit, is assembled and checked before it becomes permanent history.