I have completed torrents moved off to a NAS, but active ones are on local storage.
Sometimes when a torrent contains one file, due to (presumably) bugs in other torrent clients no peer with more than 99.99% is available.
When a torrent contains multiple files, I can uncheck them all and Transmission moves the data files to the completed location. This can't be done if there is only one file, but I've outlined a scenario in which it would make sense.