The biggest advantage for me for using Trakt instead of just relying on Plex to keep track of everything is that it will keep you up to date on new seasons, new episodes etc. Plus it will give you suggestions by seeing what others are watching. Add to that the really annoying flaw in Plex that series in your watchlist aren’t hidden once you’ve watched the current season which to be frank makes the Plex Watchlist a fairly useless feature.