The primary reason why you should have a backup copy of the
system disk is so you can fully restore your system in response
to any hardware or software problem that affects the integrity of
your original system disk or your ability to access it. For example,
you would need to use the backup copy to restore your system under the
following conditions:
When a problem occurs during an OpenVMS
Alpha upgrade or update, or during the installation of other software
products. If you backed up the system disk before you
attempted any of those procedures, you could restore the system
disk and attempt the procedure again.
When a system file that is accidentally deleted
renders the system disk inoperable. If you backed up the system
disk after you installed or upgraded the OpenVMS
Alpha operating system and any other software products, you could
restore the system disk.
When the drive that holds the system disk malfunctions.
If you have a backup copy of the system disk, you can restore it
to a functioning disk and continue to use the system.
Another reason for backing up the system disk is to eliminate
disk fragmentation, which occurs when files are stored noncontiguously
on the disk. The BACKUP/IMAGE command creates a copy on which files
are stored contiguously.
HP OpenVMS Alpha Version 7.3-2 Upgrade and Installation... 
Backing Up and Restoring the System Disk