The primary reason for having a backup copy of the system
disk is so you can fully restore your system in case any hardware
or software problem affects the integrity of your original system
disk or your ability to access it. For example, use the backup
copy to restore your system under the following conditions:
When a problem occurs during an OpenVMS upgrade
or update, or during the installation of other software products.
If you back up the system disk before you attempt any of those
procedures, you can restore the system disk and attempt the procedure
again.
When a system file that is accidentally deleted
renders the system disk nonoperational. If you back up the system
disk after you installed or upgraded the OpenVMS operating system
and any other software products, you can 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 Version 8.2-1 for Integrity Servers... 
Backing Up and Restoring the System Disk