I wrote a bit about why in the readme (see archiving vs backup). In my opinion, syncing, snapshots, and backup tools like restic are great but fundamentally solve a different problem from what I want out of an archive tool like aegis, git-annex, or boar[2].
I want my backups to be automatic and transparent, for that restic is a great tool. But for my photos, my important documents and other immutable data, I want to manually accept or reject any change that happens to them, since I might not always notice when something changes. For example if I fat finger an rm, or a bug in a program overrides something and I don't notice.