NAS vs Jellyfin isn't really how to look at it. You still need to have the files stored somewhere, and Jellyfin can just access the files from wherever you store them. As others say, Jellyfin adds some convenience. Think of it like Netflix but for your local files. You can install an app on your phone, laptop, tablet, or just access Jellyfin's built in web interface on laptop/etc... It pulls down thumbnails and show information automatically, and you can set up different accounts/profiles to track show progress and favorites for multiple people.