Who is Mr. and Mrs. Swift from The Art of Racing in the Rain: A Novel and what is their importance?

Mr. and Mrs. Swift are Denny Swift's parents, and they live on a small farm. Mrs. Swift has been blind since Denny was a child, and Denny never had a close relationship with either of his parents. Just after Denny finished high school, his father offered him an ultimatum: stay and help with caring for his mother and running the farm or leave and never come back. Denny chose to leave the farm, and his mother refused to speak to him for several years thereafter.