The primary benefit of Plex/Jellyfin over using VLC is realtime video resolution transcoding. I can save the files at 8K resolution on my NAS, then configure the output resolution depending on the client device. I can watch content at 8K on my living room TV, 4K on my laptop, and 1080p on my phone. I don't need to keep three copies of the same content at different resolutions.

Plex/Jellyfin also offer highly configurable mobile / TV companion apps. I can start watching a show on my phone, then continue where I left off on my living room TV.