At first, the whole 'Castor Clones' thing felt like a cool addition for the story of the show, now?, i can't wait for it to be over.

Means to an end, and this is as clear as it gets: "On a practical level, Kitty’s arc serves to bring Sherlock and Joan back together, his experience as her mentor putting his life into perspective and bringing him back to New York." great review for a great episode.