The api has watched_at, rated_at, and collected_at, to record when someone watched, rated or collected an item, which is particularly helpful when an app can’t sync at the exact time you want to make those changes. It would similarly be helpful if we could say when an item was added to the watchlist for the same reason.
I have my own personal reasons I’d want this feature, but I think it would be helpful to apps to match the functionality of the others.