Many people do not want to seed downloaded content forever for storage reasons. In these cases, you would download the file with your download client and leave it in that download directory to allow seeding. It'll be hardlinked to the Radarr/Sonarr folder for indexing, which does not use up extra storage space. Once a certain seeding/time goal is reached on the torrent, the torrent file will be deleted to make room for new torrents. This does (to my knowledge) not delete the file from the disk, meaning it is still accessible for your media center.

Especially for people who run their software on hosted solutions with limited storage space, this is important to do. If you have all your software running on a local server with (virtually) infinite storage, this is not as much of a worry to you. It is probably still in your best interest to use hardlinks instead of copies, to save on storage space.