The Illusory Pawn Sacrifice

White's plan is obvious: b2-b4-b5. But the b4 square is under Black's control. Possible is 1.a3 and then b2-b4-b5. But evidently the move 1.a3 will turn out to be an extra move in the further play. Much stronger looks 1.Rb1 and then b2-b4-b5. But it is quite possible that 1.Rb1 will also prove superfluous, since in the future the maneuver a2-a4-a5-a6 may be needed, and for that the rook must be on a1. So both moves, 1.a3 and 1.Rb1, threaten a loss of time, of which White do not have much anyway - Black are ready for the f7-f5 counterattack. Therefore, taking into account the undefended b7 pawn and the guarantee of a return, White played.