The computer archive service provides a long-term storage system for computer files. It complements the backup service which provides short-term backup in case of accidental deletion or disaster.
You can think of the archive as a repository or safe where you can leave things that are precious and recover them later if needed. It is intended as a write-once and then read-many copy of files that you wish to retain for a long time.
It is not a backup for everything you own and it is not intended that it should be used instead of a good backup regime for your own files on your desktop.