By not including punctuation and other grammatical structures, Cormac McCarthy creates a narrative that is starkly bare, stripped to nothingness like the novel's setting. By choosing this style for his writing, McCarthy is employing an additional vehicle, besides the content he presents, by which to communicate to the reader the emptiness of the world after the cataclysmic event that for all intents and purposes destroyed it.
It is a testimony to the authors tremendous talent that, despite his use of the bare minimum in writing his story, he is able to convey the deep love that exists between the two main characters. Through the use of the simple, unembellished statements and questions that make up their interaction and conversation, McCarthy gives the reader a clear sense of the undying tenderness and devotion that lives and grows between father and son in the face of impossible odds.
I have just finished The Road (and had read his other books earlier) and in my opinion this practice actually interferes with the clarity of the writing, in part because of the inconsistency within a given work. For example, he uses plenty of apostrophes for certain kinds of contractions but never seems to use them for a contraction of something+not. Therefore, we have "cant," "wont," etc., but also "it's," "that's," etc. I object to the inconsistency on aesthetic as well as stylistic grounds (similar problems would occur with tense shifts), but beyond that I cannot see any advantage to spelling "can't" and "won't" in a manner identical to words that mean something entirely different, since one of punctuation's functions is to disambiguate, e.g., "its" from "it's." There are ways to reduce unnecessary punctuation without resorting to an idiosyncratic practice that accomplishes little except to interpose "hiccups" in the reader's experience. Overall, his books are beautifully crafted but every time I pick one up I have to mentally hold my nose over this practice, which I consider an affectation.
Actually, his lack of punctuation and other grammatical structures (such as no semi-colons and few commas) has nothing to do with the themes in "The Road" specifically. Cormac does these same things in all of his novels such as Blood Meridian, Child of God, No Country for Old Men, and Suttree, of which I have read all of them. He said in one of his few interviews that when he was still in college he was asked to revise older writers work to make them more readable and clear. He revised (punctuation, not content) writings by Swift by severely cutting out unnecessary commas and other forms of punctuation and was praised for it. He said he always thought there was no reason for all of these marks to muck up the page. He also said that if you write well enough and clearly enough there is no reason for quotation marks. If you read interviews by him you will find out that people like to convey their own artistic impulses and ideas on him, but actually his writing is (despite the intense and esoteric vocabulary) fairly simple and straight-forward, which is what makes it so beautiful and rewarding. So, as much as people would like to think his use of punctuation (or lack) has some deeper meaning, it is actually just to make the writing less convoluted and conjested.
I quote Cormac, "James Joyce is a good model for punctuation, he keeps it to an absolute minimum. There is no reason to blot the page up with weird little marks. If you write properly you shouldn't have to punctuate...yeah punctuation is important, so that it makes it easy for people to read. I believe in periods, and capitals, and the occasional comma. You really have to be aware that there are not quotation marks to guide people though and write in a way that it is not confusing about who is speaking." So really his writing style is about simplicity and has nothing to do with "The Road" specifically.