One point to consider is that when you are taking a backup, the source and destination are both vulnerable to corruption - power failure for example.
If you are regularly backing up the contents of a disk or directory from A to B and have no C backup, both the source and backup can be damaged or destroyed if you are unlucky.
So it is prudent to either alternate backups, or backup the backup or figure out a way of avoiding having the scenario above come to pass.
SD