King doesn't remember writing the book these days (that may be his age as much as anything), but he recalled it well enough in interviews at the time, just after Cujo was published. Regarding that scene specifically, he relates writing it and shocking himself (King, as you may know, doesn't write detailed outlines for he books, just writes it as he goes) and just sitting there, thinking it over. Eventually, he decided to carry on and see where that plot point took him.

I think it makes the book—the entire thing seems to be a critique of the destruction of the nuclear family and a treatise on karma.