White’s rook masks the path of his bishop toward Black's king, so White’s task is to find a destructive role for the rook. Again the queens are off the board, and again White is picking between attacks against Black’s rook (with Re6+) and a lesser piece (with Rxb6+, attacking the bishop). Both targets are unguarded at the moment, but Black can reply to the check with Kf7, protecting the rook. So Rxb6+ is better; it wins a pawn and a bishop for nothing.