Doing a mksysb is an important sys admin task. If you get a rootvg disk failure and you aren't mirrored up, you'll need to restore your system.
Written by David Tansley
A mksysb is an AIX system backup that allows you to restore your rootvg disks to what they were at the time the backup was taken. In effect, this restores your operating system. A mksysb backup should be taken at least once a week. There's really no need to do a mksysb more often than that, as very few changes will happen on your rootvg disks.