As White you might be thinking here about your two rooks and how to coordinate them; there almost is a back rank mate with Re8+, but Black’s rook on a8 prevents it. Meanwhile your bishop on g2 might seem an afterthought, serving a largely defensive purpose since it is not aimed toward Black’s king. But in fact the bishop plays a useful offensive role, as often is the case when it is fianchettoed (sitting on g2 or a comparable square)—and especially when the center has been cleared. The bishop is pinning the pawn on b7 to the loose rook behind it. This is not beneath your notice, for it means that the pawn is incapacitated and protects nothing. So look at what it ostensibly protects: the pawn on a6. White can take it with Rxa6. If Black replies b7xR, he loses his rook; if he replies RxR, this would permit immediate checkmate by White with Re8#. So the original Rxa6 wins White a pawn.