If you have a show marked as ‘watched’ it will show up in your ‘Progress’ section, in your ‘Up Next’ and in your ‘Calendar’ when new episodes release.
The ‘Watchlist’ is only for shows you haven’t started yet, after you start them, they are registered in your ‘History’ and in your ‘Progress’, so there is no need to add them there again.
If you use the any of the apps, you can also receive notifications when new episodes are released, if you want, of course.