- [Short answer] Having a file in multiple folders without using double your storage space.
- [Long answer] Hard links are a way for a copy operation to be instant and not consume space for those additional copies. Every file is a hard link: some metadata that points at blocks on the file system, which is why they're restricted to the same file system. There can be as many metadata files pointing at those blocks as needed and the blocks know how many links point to them. Once the blocks have 0 links, they're considered deleted. That means you can delete any "copy" w/o impacting the others. Your download client can remove it's copy w/o impacting the library copy. The library copy can be removed by Plex or Sonarr/Radarr or yourself w/o impacting the download client copy. But space is only regained when all copies are deleted. Of note is that modifying the file will impact all copies. For example, modifying the id3 tags of a .mp3 download after import would modify the download client copy, breaking the torrent.