I have seen many live competitions from top GMs, and I can usually see one thinking 15 minutes straight, then doing a tactical move. After some thinking from the other player, he does the main move, the one the engines say it is the best, and probably was considered by the first GM. I keep seeing GMs thinking, spending more time, even if the opponent plays what they wanted. Double checking might be it, but still, isn't it valuable time spent on checking an already checked position?
I had the same thought myself. For a couple tournaments, I started playing lines which I had calculated out with about 10 seconds of thought on each move. My results were terrible.
The reason, I think, is that the extra move being played on the board gives you quite a bit of added calculation power. I now take more time on the moves following a long think and I frequently encounter errors in my previous analysis, which I either have to find fixes for or find a new move.
Chess has far too many possibilities to calculate. It also has time constraints, so at some point you always have to stop thinking and make a decision based on imperfect information. Barring forced moves and forced wins, you always have imperfect information and so you have to rely on judgment all the time.
When Carlsen was considering Bxf7, he had to think about several possible replies, and of course he also had to spend time on the other moves he could have played and the possible replies to those. After fifteen minutes he decided that Bxf7 looked best so far, spending more time wasn't productive and he went for it. That doesn't mean the position after Kxf7 is "already checked" in any sense of the word.
Now that he can concentrate only on the position after Bxf7 Kxf7, and not on all the other positions that might have occurred, it makes sense to spend more time to look deeper.
A lot of times, Grandmasters will not make concrete plans. The plan might be general - "I have to attack the enemy king", without having any specific move order in mind. Also, many times grandmasters make moves based purely on intuition than on concrete analysis. It might seem to the observers that the grandmaster has been "planning", but it may just be that all this while he/she was simply making sure their intuition was sound, or was even occasionally distracted by some irrelevant thought.
Mikhail Tal notes one such famous incident in his own life (emphasis mine)-
I will never forget my game with GM Vasiukov on a USSR Championship. We reached a very complicated position where I was intending to sacrifice a knight. The sacrifice was not obvious; there was a large number of possible variations; but when I began to study hard and work through them, I found to my horror that nothing would come of it. Ideas piled up one after another. I would transport a subtle reply by my opponent, which worked in one case, to another situation where it would naturally prove to be quite useless. As a result my head became filled with a completely chaotic pile of all sorts of moves, and the infamous "tree of variations", from which the chess trainers recommend that you cut off the small branches, in this case spread with unbelievable rapidity.
And then suddenly, for some reason, I remembered the classic couplet by Korney Ivanović Chukovsky: "Oh, what a difficult job it was. To drag out of the marsh the hippopotamus". I do not know from what associations the hippopotamus got into the chess board, but although the spectators were convinced that I was continuing to study the position, I, despite my humanitarian education, was trying at this time to work out: just how WOULD you drag a hippopotamus out of the marsh? I remember how jacks figured in my thoughts, as well as levers, helicopters, and even a rope ladder.
After a lengthy consideration I admitted defeat as an engineer, and thought spitefully to myself: "Well, just let it drown!" And suddenly the hippopotamus disappeared. Went right off the chessboard just as he had come on ... of his own accord! And straightaway the position did not appear to be so complicated. Now I somehow realized that it was not possible to calculate all the variations, and that the knight sacrifice was, by its very nature, purely intuitive. And since it promised an interesting game, I could not refrain from making it. And the following day, it was with pleasure that I read in the paper how Mikhail Tal, after carefully thinking over the position for 40 minutes, made an accurately calculated piece sacrifice.
— Mikhail Tal, The Life and Games of Mikhail Tal.
Here's the game for your viewing pleasure. The knight sacrifice is on move 19.
[FEN ""] [White "Mikhail Tal"] [Black "Vasiukov"] 1. e4 c6 2. Nc3 d5 3. d4 dxe4 4. Ne4 Nd7 5. Nf3 Ngf6 6. Ng3 e6 7. Bd3 c5 8. O-O cxd4 9. Nd4 Bc5 10. Nf3 O-O 11. Qe2 b6 12. Bf4 Bb7 13. Rad1 Nd5 14. Bg5 Qc7 15. Nh5 Kh8 16. Be4 f6 17. Bh4 Bd6 18. c4 Ba6 19. Ng7 Kg7 20. Nd4 Nc5 21. Qg4 Kh8 22. Ne6 Ne6 23. Qe6 Rae8 24. Qd5 Bh2 25. Kh1 Qf4 26. Qh5 Qe4 27. Rfe1 Qg6 28. Qg6 hxg6 29. Bf6 Kg8 30. Re8 Re8 31. Kh2 Bc4 32. Rd7 Re6 33. Bc3 Ba2 34. Ra7 Bc4 35. Kg3 Bd5 36. f3 Kf8 37. Bd4 b5 38. Kf4 Bc4 39. Kg5 Ke8 40. Ra8 Kf7 41. Ra7 Ke8 42. b4 Bd5 43. Ra3 Kf7 44. g4 Re2 45. Bc5 Re5 46. Kh6 Re6 47. Rd3 Bc6 48. Rd8 Re8 49. Rd4 Re6 50. f4 Ke8 51. Kg7 Be4 52. Bb6 Bf3 53. Rd8 Ke7 54. Rd3 Be2 55. Bd8 Ke8 56. Rd2 Re3 57. Bg5 Bd3 58. f5