You don’t use it, the system might in edge cases

I have had items put there a few time on an fsck. Not often but it has happened.

I think the most common use case was when a file descriptor was still open in some process, but the file itself had been completely unlink'd from the filesystem. Most often it was some sort of temporary file.

Anyway the /lost+found link would be its inode number, I think. And then you could recover it after fsck had relinked it into the hierarchy. It would have to put them there, because it had no idea what pathname it had used when it was a real file.