White's development advantage has to be exploited as fast as possible, before black can catch up. The exchange sac g5 looks very strong to me. If Bxh1 gxf6, I don't see how black can defend against the long castling coming as well as the queen. All white's pieces are active against a shattered castle structure. And if black declines the sac with Nc6, hoping to exchange some pieces, then f3 wins a piece.