Because after 1.Rg3 Kh8 2.c3 Nxb3 3.axb3 Bxd5 white must take on d5 with the pawn and black is totally ok as seen in the game, while after 1.c3 Nxb3 (1...c4 2.Rg3 doesn´t win a piece and is good for white as Robert shows) 2.axb3 Bxd5 white would advantageously take on d5 with the rook showing the advantage of keeping Rg3 in reserve.

@@marcnarciso6838 Thank you. It’s just that I believe the 1. c3 c4 2. Rg3 Bxd5 line leads to a draw, which would be fine for black. So playing c3 before Rg3 doesn’t really gain an advantage for white, since black would just play c4 anyway and secure the half a point.

​@@omenoflaherty1294Looks indeed like a perpetual after 3.Q:h6 g6 4.e:d5 c:b3 5.R:g6+ .