>> They are generated by _newfs_ and left untouched thereafter.
> Interesting, thanks. what's so useful about the superblock at newfs
> time?
It contains enough information for fsck to find other critical things
(like cylinder groups and their inode tables). If the primary
superblock has been destroyed but the rest of the filesystem is intact,
fsck is supposed be able to put the filesystem back together with the
help of a backup superblock.