Think of Jellyfin as a multi-user, centralized version of Kodi. Thanks to this, you get the ability to access (and give others access to) your Jellyfin instance remotely from virtually any device.

Kodi can serve as either a Jellyfin client or as a local replacement. One thing to note is that the Kodi plugin ecosystem is much larger than that of Jellyfin, or any other media server solution for that matter.