White has an inviting prospect here for a pawn fork: Black’s king and rook are a square apart on the same rank, and White can reach them with g2-g4+. (Naturally you might as well have found this by just looking for White’s checks.) True, the pawn has no protection; but it may have value anyway just as a forcing move. If Black retreats his king he loses his rook, so he would instead need to play Kxg4. Imagine the result and you see that his king and rook would end up on the same light-squared diagonal. Time to go to work with the light-squared bishop: White plays Be6, pinning the rook. He takes it a move later with BxR, winning the exchange.