In Fischer-Petrosian 1971 game 7, why did Fischer give up his good knight for a bad bishop? Fischer played 22. Nxd7, which seems strange since the knight looked very good. Why did he play it?

 [Title "Fischer-Petrosian, Buenos Aires (7) 1971, 10/19/1971"]
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  1. e4 c5 2. Nf3 e6 3. d4 cxd4 4. Nxd4 a6 5. Bd3 Nc6 6. Nxc6 bxc6 7. O-O d5 8. c4 Nf6 9. cxd5 cxd5 10. exd5 exd5 11. Nc3 Be7 12. Qa4+ Qd7 13. Re1 Qxa4 14. Nxa4 Be6 15. Be3 O-O 16. Bc5 Rfe8 17. Bxe7 Rxe7 18. b4 Kf8 19. Nc5 Bc8 20. f3 Rea7 21. Re5 Bd7 22. Nxd7+ Rxd7 23. Rc1 Rd6 24. Rc7 Nd7 25. Re2 g6 26. Kf2 h5 27. f4 h4 28. Kf3 f5 29. Ke3 d4+ 30. Kd2 Nb6 31. Ree7 Nd5 32. Rf7+ Ke8 33. Rb7 Nxf4 34. Bc4 1-0
This is a very famous position. Well, here are some reasons, and there are quite a few:


Since this was a Candidate’s Match to qualify to play Spassky, it comes down to exact calculation above all. Fischer calculated that it was good, and his judgement bore out since the game only lasted another 12 moves. Here are some things that probably contributed to this judgement.
It was clear that black had to take with the Ra7 so as not to lose d5, and after that, neither black rook remains well placed since the Ra8 cannot leave the Pa6 undefended, and either the Rd7 or the Nf6 is stuck defending d5. D5 also limits the scope of the Rd7, in particular. 
It also leaves the Pa6 particularly weak since the Bd7 might have been able to defend it. GM Mikhail Suba is famous for saying "Bad bishops defend good pawns!" In addition, when you look at the minor pieces that are left, the Bd3 versus the Nf6 is clearly in favor of the B and its nice scope. It is of note that if black tries to liquidate the Pa6 weakness with a5, white can just push, creating a dangerous passer.
Lastly, and most importantly in this position, the white Ra1 immediately takes the only useful file, the c-file, with an immediate threat of Rc6, thus black played Rd6 stopping it, which allowed white to take the 7th rank.
In all fairness, the computer finds both 22.a4 and 22.Rc1 stronger by about half a pawn, but Fischer’s method is clear, and very straightforward.