What is the significance of dreams and flashbacks about the woman in Cormac McCarthy's The Road?

In the post-apocalyptic world depicted in Cormac McCarthy's The Road, pleasant memories of dead loved ones are a luxury this story's protagonist, the man, can ill-afford. McCarthy's depiction of a world destroyed and the depths to which survivors must descend to continue to survive is innately bleak. There is no hope for the man and his son. All they can do is survive by any means necessary, and sentiments can prove deadly, a display of weakness that can increase one's sense of vulnerability. This is a world in which cannibalism is common, and absolute ruthlessness a sine qua non of survival. The man dreams of a woman, his wife and the mother of the boy. The man's mission in life now is to prepare his son for the incomprehensibly brutal, unforgiving world in which he must grow up. 
While the man must be pragmatic and cold, his brain contains memories of the wife he lost in the catastrophic events that destroyed civilization. Early in The Road, McCarthy emphasizes the limitations under which the man must live in order to keep his edge, and those limitations preclude loving images:

"In dreams his pale bride came to him out of a green and leafy canopy.... She wore a dress of gauze and her dark hair was carried up in combs of ivory, combs of shell. Her smile, her downturned eyes. In the morning it was snowing again....
He mistrusted all of that. He said the right dreams for a man in peril were dreams of peril and all else was the call of the languor and of death. He slept little and he slept poorly. He dreamt of walking in a flowering wood where birds flew before them he and the child and the sky was aching blue but he was learning how to wake himself from just such siren worlds."

This passage is only the first in The Road to reference the woman. It is not, of course, the last. The boy's mother is dead, by her own hand, and the man does not have the luxury of mourning her death or dreaming about his life with her. Another passage in the opening pages of McCarthy's novel suggests the man's sense of guilt over his wife's death: "He did not take care of her and she died alone somewhere in the dark and there is no other dream nor other waking world and there is no tale to tell."
As noted above, the world McCarthy depicts does not allow for empathy, nor does it allow for beauty. McCarthy's story features a society in which memories of beauty are reserved for very few, and not for a man charged with preparing his son for the struggles that lie ahead. The significance of the dreams and flashbacks about the woman are to remind the protagonist and the reader of a world that no longer exists and of the man's continuing struggle to harden himself and his son for the realities in which they live.