Intention of two moves of Topalov vs Anand

Question

What was the intention for the moves 23. ... Bb6 and 24. ... Bd7 ?

[FEN ""]
[White "Anand"]
[Black "Topalov"]
[StartPly "45"]

1. d4 Nf6 2. c4 e6 3. Nf3 d5 4. g3 dxc4 5. Bg2 a6 6. Ne5 c5 7. Na3 cxd4 8. Naxc4 Bc5 9. O-O O-O 10. Bg5 h6 11. Bxf6 Qxf6 12. Nd3 Ba7 13. Qa4 Nc6 14. Rac1 e5 15. Bxc6 b5 16. Qc2 Qxc6 17. Ncxe5 Qe4 18. Qc6 Bb7 19. Qxe4 Bxe4 20. Rc2 Rfe8 21. Rfc1 f6 22. Nd7 Bf5 23. N7c5 Bb6 24. Nb7 Bd7 25. Nf4 Rab8 26. Nd6 Re5 27. Nc8 Ba5 28. Nd3 Re8 29. Na7 Bb6 30. Nc6 Rb7 31. Ncb4 a5 32. Nd5 a4 33. Nxb6 Rxb6 34. Nc5 Bf5 35. Rd2 Rc6 36. b4 axb3 37. axb3 b4 38. Rxd4 Rxe2 39. Rxb4 Bh3 40. Rbc4 Rd6 41. Re4 Rb2 42. Ree1 Rdd2 43. Ne4 Rd4 44. Nc5 Rdd2 45. Ne4 Rd3 46. Rb1 Rdxb3 47. Nd2 Rb4 48. f3 g5 49. Rxb2 Rxb2 50. Rd1 Kf7 51. Kf2 h5 52. Ke3 Rc2 53. Ra1 Kg6 54. Ra6 Bf5 55. Rd6 Rc3+ 56. Kf2 Rc2 57. Ke3 Rc3+ 58. Kf2 Rc2  1/2-1/2

Answer 1

Well I'm no GM but for me there's a couple of things going on.

1. In the given position white is threatening to take the pawn on a6
2. If white can get a knight to d6 he will fork the rook on e8 and the bishop on f5, so winning the exchange after a move like Bd7

Thus black's move 23 protects the pawn on a6 by uncovering the line of the rook on a8. White then tries to get his knight to d6 via b7, and black stops the fork by moving his white squared bishop back to d7.

Answer 2

Alongside @IanBush's answer, the bishops prevent white's doubled rooks from penetrating black's camp:

[FEN ""]
[White "Anand"]
[Black "Topalov"]
[StartPly "48"]

1. d4 Nf6 2. c4 e6 3. Nf3 d5 4. g3 dxc4 5. Bg2 a6 6. Ne5 c5 7. Na3 cxd4 8. Naxc4 Bc5 9. O-O O-O 10. Bg5 h6 11. Bxf6 Qxf6 12. Nd3 Ba7 13. Qa4 Nc6 14. Rac1 e5 15. Bxc6 b5 16. Qc2 Qxc6 17. Ncxe5 Qe4 18. Qc6 Bb7 19. Qxe4 Bxe4 20. Rc2 Rfe8 21. Rfc1 f6 22. Nd7 Bf5 23. N7c5 Bb6 24. Nb7 Bd7 {[%draw full,c4,red][%draw full,c5,red][%draw full,c6,red][%draw full,c7,red][%draw full,c8,red][%draw arrow,b5,c4,green][%draw arrow,b6,c5,green][%draw arrow,b6,c7,green][%draw arrow,d7,c6,green][%draw arrow,d7,c8,green]} 25. Nf4 Rab8 26. Nd6 Re5 27. Nc8 Ba5 28. Nd3 Re8 29. Na7 Bb6 30. Nc6 Rb7 31. Ncb4 a5 32. Nd5 a4 33. Nxb6 Rxb6 34. Nc5 Bf5 35. Rd2 Rc6 36. b4 axb3 37. axb3 b4 38. Rxd4 Rxe2 39. Rxb4 Bh3 40. Rbc4 Rd6 41. Re4 Rb2 42. Ree1 Rdd2 43. Ne4 Rd4 44. Nc5 Rdd2 45. Ne4 Rd3 46. Rb1 Rdxb3 47. Nd2 Rb4 48. f3 g5 49. Rxb2 Rxb2 50. Rd1 Kf7 51. Kf2 h5 52. Ke3 Rc2 53. Ra1 Kg6 54. Ra6 Bf5 55. Rd6 Rc3+ 56. Kf2 Rc2 57. Ke3 Rc3+ 58. Kf2 Rc2  1/2-1/2

Answer 3

...Bb6 because the a6 pawn is hanging; ...Bd7 because of the threat of Nd6 forking the Re8 and Bf5. Black wants to keep the bishop pair.