[derefr]

There's still the problem of Time Machine (or any other backup software you use) needing to do a complete deep scan of the volume to ensure you haven't made any changes. If you know a git repo is effectively in a "read-only" state—something you just keep around for reference, or an old project you might "get back to" a few years from now—it can speed up those backups dramatically to put the repo into some kind of archive. Disk images, for this use-case, are just archives you can mount :)