If you have one client - nothing. Kodi does more than Plex in this scenario.
And just to add, with a very simple little config file, and an SQL database sitting on your NAS, you can achieve central storage of play/paused info, meta data etc within Kodi if you do add another Kodi player in the future.
Before you jump in and buy a Plex pass it's worth taking a look at Jellyfin as an alternative.
It started as a free, open-source fork of Emby and has really come into it's own with the last couple of releases.
May not be what you are after but definitely worth a look.
I couldn't recommend it more.
If you prefer Kodi on some devices (TV's) you can also sync Kodi clients to a shared Jellyfin/Emby server.