The flour-enriched water in which pasta has cooked is never
going to be an essential component of fine cooking, and it seldom appears in recipes. Yet from its origins as a cost-free, effortless substitute for stock, olive oil, butter, cream, or other occasionally scarce or even precious ingredients, pasta-cooking water has become a convenient and zero-calorie addition to simple sauces.
When you compare a lightly creamy sauce like the one in this recipe to the highly flavorful and ever-popular Alfredo sauce of butter, cream, eggs, and cheese, the latter seems relatively heavy. Substituting water for much of the butter and all of the cream and eggs produces a sauce with a perfect balance of weight and flavor. The water lends a quality not unlike that produced by tomatoes, as opposed to the slickness contributed by straight fat.