This blessing thanks G‑d for creating man; Adam. This blessing is a precursor to the following one, which thanks G‑d for creating Eve, thus allowing for the possibility of marriage.
This blessing, however, has deep meaning on its own. Before Eve was surgically removed from Adam's own flesh, she and Adam were one entity, one being. Spiritually, too, the souls of every husband and wife were originally one entity in the Garden of Eden, before being sent down to inhabit the respective bodies of a man and woman. The ability to physically and emotionally fuse a male and female, to make two into one, stems from their souls' original state of oneness. This blessing, which alludes to the point in time when man and woman were still one entity, is intended to evoke the soul connection within the context of this physical union.
Birkat Hamazon