[%c_effect b2;square;b2;type;GreatFind;persistent;true] This is the idea: the rook prepares to travel to d2 and defend the d4-pawn, thus enabling the e3-e4 advance. This idea is known from many openings, including the Nimzo-Indian itself, although the rooknormally travels from a2. This is one of the secrets of high-level chess—using similar ideas in different positions. Giri used the same pattern in his excellent win against Gukesh in the Tata Steel tournament.
1. d2d4  1... g8f6  2. c2c4  2... e7e6  3. b1c3  3... f8b4  4. e2e3  4... e8g8  5. f1d3  5... d7d5  6. a2a3  6... b4c3  7. b2c3  7... d5c4  8. d3c4  8... c7c5  9. g1e2 The advantage of this move over the more natural development of the knight on f3 is that it can now go to g3, supporting the advance of the e-pawn. 9... d8c7  10. c4a2  10... b7b6  11. e1g1  11... c8a6 An excellent diagonal for the bishop. 12. f1e1  12... b8c6  13. e2g3  13... f8d8  14. a1b1 This move was a novelty by Caruana, and the idea becomes clear in a moment. 14... a8c8 