Starches thicken sauce in a way that's quite different from reducing. Most Italian sauces end up adding some of the pasta water back in at the end. This is sta

Starches thicken sauce in a way that's quite different from reducing. Most Italian sauces end up adding some of the pasta water back in at the end. This is starchy water which has two purposes: both to loosen (thin) the sauce, making it spread out to cover the whole dish, and to thicken the sauce by giving it a slightly gluey texture (unappetizing description, I know) that helps it bind together and stick to the pasta. These two purposes sound like opposites but are not. A tomato sauce that has simply reduced will be both dense and not bind well to the pasta. (To be sure, reducing also builds flavor, which is why you first reduce and then add starch water.)