Jellyfin user here, glad I dodged the bullet when I had to pick between it and plex.

Tl;dr you want something like plex to:

• manage your media files for you
• get metadata for extra features (eg. show me similar movies, select an actor from the cast and see all your media with that actor, etc)
• track your watch progress
• play on several devices (tv, mobile, pcs consoles)
• transcode media to a compatible format for your client device
• share your media library with your family
• get notified of related media being released (new season of a show or new movie onba series)

And the biggest one for me

• tidy up ripped dvd/br movie collection, download missing CC or subtitles
• create a self-hosted alternative to shitty subscription services