The most important purpose of these first positions is just to train your eyes to see a single pattern: a bishop masking an attack by a rook or queen on the same file. Here the pieces are farther apart than before, but the pattern is structurally the same. White’s job is to move the bishop out of his rook's way violently—preferably with a check. Bg8+ does the job: another example of a less usual check, as the bishop attacks the king from the rear. White will play RxR next move, winning the exchange.