Restore from lost+found

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 lost+found directory, 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.

i also notice link count is very suspicious (nearly 2^32).

you can try more recent e2fsck, and/or report a bug. this is certainly a bug.

scanning the device/partition with photorec might restore more files, where the format is supported and they are contiguous. since your FS is quite full, many files are not contiguous. photorec does not recover file names or directories. (e.g. if they're mp3's, you can use something like picard to apply filenames from the mp3 metadata aka ID3 tags). note photorec requires free space on another filesystem, to recover all the files to.