use case: The Archivist

Bob has many drives to archive his data, most of them kept offline, in a safe place.

With git-annex, Bob has a single directory tree that includes all his files, even if their content is being stored offline. He can reorganize his files using that tree, committing new versions to git, without worry about accidentally deleting anything.

When Bob needs access to some files, git-annex can tell him which drive(s) they're on, and easily make them available. Indeed, every drive knows what is on every other drive.
more about location tracking

Bob thinks long-term, and so he appreciates that git-annex uses a simple repository format. He knows his files will be accessible in the future even if the world has forgotten about git-annex and git.
more about future-proofing

Run in a cron job, git-annex adds new files to archival drives at night. It also helps Bob keep track of intentional and unintentional copies of files, and logs information he can use to decide when it's time to duplicate the content of old drives.
more about backup copies