It also allows easier switching between multiple Jellyfin servers or users since it doesn't have to rely on syncing all the metadata down. Media is accessed primarily by going through the Add-ons -> Jell圜on menu, however you can set up menu options to link to it and show info on the home screen.
The sync process can take some extra time on Kodi startup if you don't leave it running 24/7, but it's mostly in the background while Kodi is running. However, it also tends to consume the database and not share well, so if you have local media or something else that interacts with the database directly, you'll have conflicts and it won't be happy. This has the effect of making interacting with it feel very much like vanilla Kodi with local media (shows up under Movies/TV Shows on the home screen by default, virtually no delay, etc).
There are two different Kodi add-ons that serve slightly different use cases.