I’d like to track these things:

• movies/shows I’d like to see at some point
• movies/shows I have in my collection
• movies/shows I have watched (and when/how often)
  • and they should automatically get removed from the watchlist once watched

I believe TMDB only does the first. And self-hosting makes sure the data stays under my control and the service doesn’t vanish or gets paywalled anytime soon.