A typical plan in these positions, demonstrated in several games by Almasi and Kramnik, the two main precursors of the modern Berlin. This move serves several functions, both preventing the advance of the white pawn majority on the kingside, and preparing the activation of the rook after h4-Rh5.
1. e2e4  1... e7e5  2. g1f3  2... b8c6  3. f1b5  3... g8f6  4. e1g1  4... f6e4  5. d2d4  5... e4d6  6. b5c6  6... d7c6  7. d4e5  7... d6f5  8. d1d8  8... e8d8  9. b1c3 Gukesh joins the team of elite players who are still trying to break the Berlin Defense by playing the endgame. If I'm not mistaken, this is the first time he's taken this path. 9... d8e8 Black needs to decide whether to bring the king to the kingside (the most solid plan) or to the queenside (the most ambitious plan). Firouzja chooses the former, but his games always go for wild complications sooner or later. 10. h2h3  10... f5e7  11. f1d1 The computer prefers to place the rook on e1, but this is certainly part of many hours of preparation by the Gukesh team, analyzing every detail to pose practical problems for Black. A possible explanation for the rook's choice of square is that after 11... e7g6  12. b2b3  12... f8e7  13. c1b2 