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it resolves lots of loose strings from the original trilogy.
Closing the huge plot WTF of "A New Hope" was resolving a loose transatlantic cable. It's funny, because if @Light_EDDed ever decides to watch it, there's a string of moments where they'll be just "Oh, yes, that's because of X" where several generations of Star War fans have been saying "Um, OK, forget that plot hole, we all know, it's still a great film".
Rogue One is awesome. It’s the first ‘adult’/darker Star Wars movie, stands pretty well on its own, and it resolves lots of loose strings from the original trilogy.