White obviously has great power on the h-file, but he doesn't quite have the diagonal support he needs to mate because Black’s bishop on d4 blocks the way. If White plays BxB, Black has QxB. Can Black’s bishop instead be dragged away from its position? It can, with a check sacrifice. White plays Qh8+, requiring the reply BxQ; and now comes RxBh8#. This is a helpful general idea: if the path of your bishop to h8 is blocked only by an enemy bishop on the same diagonal, it may be possible to eliminate the bishop with a sacrifice on the mating square. Even more generally, when you are trying for a mate that can be executed with a bishop and rook and you have those pieces available, your queen may be expendable—and may be very useful to sacrifice.