Their original purpose was to help administrators identify unused files to migrate to auxiliary storage. This is discussed in a BtrFS post on the atime impact

Their original purpose was to help administrators identify unused files to migrate to auxiliary storage. This is discussed in a BtrFS post on the atime impact to snapshots (quite harmful). Definitely mount BtrFS with noatime when snapshots are in use (likely best with ZFS also). https://lwn.net/Articles/499293/ https://lwn.net/Articles/499293/