Jellyfin and plex are basically just GUIs to make playing videos from your NAS feel like you’re using a streaming service. They do a bunch more stuff, but in general that’s what they’re for.

Like, you don’t need a GUI for Linux, but it can certainly make the experience better.

The files are still stored on a NAS.

But, Jellyfin/Plex has the advantage you get a nice pretty “app” that works on your TV/Roku/AndroidTV/etc. It handles transcoding if needed, keeps track of what you have watched, and lets you know when new things pop up.

I use both plex and jellyfin and my files are on a nas. Previously truenas but now synology. I just mount my collection over smb to my Intel nuc with quick sync so that Plex/jellyfin can provide me and my friends a slick UI as well as transcoding (can store stuff in hevc, flac, 5.1 or 7.1 dts hd ma and not worry about codec support on each device), a nice web player with subtitles /audio track selection, and nice apps on every device to access the collection.

But yeah NAS and jellyfin aren’t mutually exclusive, many people use them together.

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.

Jellyfin/Plex/Emby turn your media collection into your own personal Netflix. They have apps for multiple platforms, you can setup user access controls, parental controls based off age ratings, track progress through shows and movies, search and organize based off genre and tags, and much more. Also, they can handle on the fly transcoding of the media, so if a device doesn’t support a specific codec or container it can be converted into another, or if the user is on a poor Internet connection which can’t handle a 4K video, it can downgrade the quality to make it easier to stream on the poor connection.

Overall, they just provide a better experience when consuming media.

It keeps track of which files you’ve played (e.g. to automatically pick the next episode in a series), it automatically downloads metadata and cover art so you have a nice browsing interface, it manages multiple profiles so that e.g. you can limit your kids’ access to only G and TV-Y or filter out genres a user doesn’t like, it lets you set parental controls to limit the amount of time watched in a day (or disable it at certain times of day), etc.

A slick interface with nice title cards and pictures, feels like your own personal streaming service with no drawback