There are two main reasons for this. I'm not going to lie to you, the number one reason is so there are fewer dishes to wash. If you'd rather make the pasta separately or have leftover pasta you want to jazz up with this, you certainly can. The other reason is that instead of needing to reserve a cup of pasta water we are keeping all of that starchy water right in the pan with the pasta which makes it feel a bit thicker. With the parmesan added it takes on a bit of creaminess and while it's not a true creamy sauce, it's on the thicker side for a recipe that doesn't use heavy cream.