10
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]

1. c4 g6 2. d4 Nf6 3. Nc3 d5 4. Nf3 Bg7 5. cxd5 Nxd5 6. e4 Nxc3 7. bxc3 c5 8. Be3 Qa5 9. Qd2 Nc6 10. Rb1 a6 11. Rc1

In this line of the Grunfeld, White has played 10. Rb1 followed by 11. Rc1. Why? If White wants the rook on c1, why not just play 10. Rc1? What is the benefit of provoking 10...a6 first?

8

First, and you probably know this, but for other people reading this, I should spell this out: White is playing on the queenside and in the center, and black is trying to chip away at that center to permanently weaken it. Most of the play by white here is very concrete and prophylactic, specifically, trying to prevent black from breaking up the center. A lot of this has been developed over decades of grandmaster play.

Thus, in general, many moves by black on the queenside help white make inroads by giving up squares. For example, a6 and b5 is often met by a4 at some point, tearing the queenside apart for the white pieces where they are much more active. By threatening Rb5, white either forces cxd4 immediately, or the slight weakening of the b6 square. That is significant since the Rf8 often comes to d8, and Bb6 is then a potential threat. Both lines are given below, and the subtle 10.Rc1 gives white a slightly better chance at an advantage. I followed the main lines that were played the most in ChessBase's Mega 2020 database.

Now compare these two main lines with 10.Rc1, and then with almost the same line after 10.Rb1 b6 and then 11.Rc1. I think they will shine light on how forcing that subtle weakness makes a difference in black's options.

 [FEN ""]

 1. c4 g6 2. d4 Nf6 3. Nc3 d5 4. Nf3 Bg7 5. cxd5 Nxd5 6. e4 Nxc3 7. bxc3 c5 8. Be3 Qa5 9. Qd2 Nc6 10. Rb1 {With the threat of Rb5.} (10. Rc1 cxd4 11. cxd4 Qxd2+ 12. Kxd2 O-O 13. d5 Rd8 14. Ke1 Na5 15. Bg5=) 10... a6 (10... cxd4 11. cxd4 O-O 12. Qxa5 Nxa5 13. Bd3 Rd8 14. Ke2 Bg4 15. d5 e6 16. Bg5 f6 17. Bd2 $14) 11. Rc1 cxd4 12. cxd4 Qxd2+ 13. Kxd2 O-O 14. d5 Rd8 15. Ke1 Na5 $4 (15... Nb4? 16. Bd2 $1 a5 (16... Nxa2 17. Rc2 $18) 17. a3 Na6 18. Bxa5 $16) (15... Ne5 16. Nxe5 Bxe5 17. f4 $14 (17. Bb6 $5)) 16. Bb6 $18 {This dramatically shows how the weakening of b6, by means of the subtle 10.Rb1 first, changed black's options.}
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10

Peter Svidler explains this in his excellent video series and e-book on the Grünfeld (Premium access only).

The point is the initiative. This move discourages black from playing 11...cxd4, because that would lead to a very uncomfortable situation for black: After 12.cxd4 Qxd2+ 13.Kxd2 O-O 14.d5 Rd8 15.Ke1, there is no good square for the knight. 15...Na5 is now impossible, because the bishop can go to b6.

And this is the reason why Svidler follows Avrukh's recommendation to play 11...Bg4 in this case.

[ECO "D85"]
[Opening "Grünfeld Defense: Exchange Variation"]
[FEN ""]

1. d4 Nf6 2. c4 g6 3. Nc3 d5 4. cxd5 Nxd5 5. e4 Nxc3 6. bxc3 Bg7 7. Be3 c5 8. Nf3 Qa5 9. Qd2 Nc6 10. Rb1 a6 11. Rc1 cxd4 (11... Bg4 { Recommended by Avrukh and Svidler } 12. d5 Rd8) 12. cxd4 Qxd2+ 13. Kxd2 O-O 14. d5 Rd8 15. Ke1 Nb4 (15... Na7 { Best move according to Stockfish. Has never been played. })  (15... Ne5 16. Nxe5 Bxe5 17. f4 Bb2 18. Rc2 Ba3)(15... Na5?? 16. Bb6!) 16. Bd2 a5 17. a3 Na6 *
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