Why would you ever need to run this command?
In case the
lost+found directory doesn't exist. Since it's just an ordinary directory, the
root user can remove it using
rm -r. Some versions of
fsck, when they need to make use of a
lost+found directory, will create it if it doesn't exist, and some versions won't. If there's no
fsck can't recover orphaned files, that is, files that do not have any directory entries that refer to them.
The Linux version of
mklost+found has the following feature (from the mklost+found man page):
mklost+found pre-allocates disk blocks to the lost+found directory so that when e2fsck(8) is being run to recover a filesystem, it does not need to allocate blocks in the filesystem to store a large number of unlinked files. This ensures that e2fsck will not have to allocate data blocks in the filesystem during recovery.
This means that, if you have to recover files from a damaged filesystem using
fsck, fewer files will be lost as part of the recovery process because
fsck won't need to allocate blocks from the filesystem; such blocks which may contain valid file data.
What would happen if you have multiple lost and found directories?
For a given filesystem,
fsck will only use one
lost+found directory: the one that is at the filesystem's root directory. Any other
lost+found directory will not be treated specially.