used for "image" backups ;-)
for backing up "raw" volumes (or any logical volumes...)
Say you have a file system with only 10 GB but it has a million files in
it... the overhead in backing up a million files is BAD but you could
perform an image backup of the logical volume on which the file system
resides and it would be much quicker due to the lesser amount of overhead.
Granted you have to restore it as a whole but...

so that is what it is used for.

should have seen it earlier though.

Dwight