Why did Fischer play 17.c4 here? Why did Fischer play 17...c4 here?

[fen "r4rk1/1q1n1pp1/4b2p/p1pp4/8/Q2NP3/PP2BPPP/R4RK1 b - - 0 1"]


Doesn’t it make the bishop e6 a lot worse? I would have played rfb8 instead, but I am not Fischer.
Every move has pros and cons. Move a pawn, for example, and you gain space, and control squares; but you also weaken squares. The trick is to accurately determine if you will be able to use your pros better than your opponent can use your new-found weaknesses against you. Always remember that a “weakness” is only a weakness if your opponent can get at it, and use it against you.

In this position, 17…Rfb8 is also a very good move, but Stockfish definitely prefers c4. After 17…c4, Fischer decided that the immediate pressure on b2 was more important than making the bishop worse. In addition, when the Nd3 moved to f4, from there, it really could only trade itself off for that bad bishop in most situations. As in the game, because white did not take immediately on e6, the bishop went to f5, and deprived the white rooks of the b1 square, and thus, a method of defending the pawn on b2.

It was almost inevitable that white would lose a pawn. As it was played, after 21…g5, white felt compelled to give up the piece altogether seeing the variation 22. Nh5 Ne4 23. Rc2 Qb4 paralyzing white.

I do not know if you are reading “My 60 Memorable Games”, but this is game 34 in that book, and the game was titled, “Hanging pawns unhung”. Lastly, it is interesting to note that Fischer thought that the best move was 19. Nxe6 getting rid of the “bad” bishop, giving the variation: 19. Nxe6 fxe6 20. Bg4 Ra6! 21. b3! cxb3 22. axb3 Qxb3 23. Qe7 Nf8 24. Ra3 Qb4 25. Qxb4 Rxb4 26. Be2 Ra7 27. Rfa1 a4 28. Bd1 “with good drawing chances”, but 21…Qb4! Stopped the queen from taking up the active position deep in the black camp, and threatens cxb3 with a strong passed pawn, since it exposes an attack on the hanging Bg4. Per Stockfish Qxb4 is forced, and is probably close to lost in my opinion due to the incredibly weak white queenside.

 [Event "Interzonal-05"]
 [Site "Stockholm"]
 [Date "1962.03.04"]
 [Round "22"]
 [White "Bertok, Mario"]
 [Black "Fischer, Robert James"]
 [Result "0-1"]
 [ECO "D59"]
 [PlyCount "62"]
 [FEN ""]

 1. d4 d5 2. c4 e6 3. Nc3 Be7 4. Nf3 Nf6 5. Bg5 O-O 6. e3 h6 7. Bh4 b6 8. cxd5 Nxd5 9. Bxe7 Qxe7 10. Nxd5 exd5 11. Be2 Be6 12. O-O c5 13. dxc5 bxc5 14. Qa4 Qb7 15. Qa3 Nd7 16. Ne1 a5 17. Nd3 c4 18. Nf4 Rfb8 19. Rab1 (19. Nxe6 fxe6 20. Bg4 Ra6 $1 21. b3 $1 {Fischer's !} cxb3 (21... Qb4 $1 {Not letting the Qa3 get active.} 22. Qxb4 axb4 {And white is in huge trouble.}) 22. axb3 Qxb3 23. Qe7 Nf8 24. Ra3 Qb4 25. Qxb4 Rxb4 26. Be2 Ra7 27. Rfa1 a4 28. Bd1 $15) 19... Bf5 20. Rbd1 Nf6 21. Rd2 g5 22. Nxd5 (22. Nh5 Ne4 23. Rc2 Qb4 {Winning positionally.}) 22... Nxd5 23. Bxc4 Be6 24. Rfd1? Nxe3! 25. Qxe3 Bxc4 26. h4 Re8 27. Qg3 Qe7 28. b3 Be6 29. f4 g4 30. h5 Qc5+ 31. Rf2 Bf5 0-1