Pasta water is a key ingredient in many Italian dishes like this. It adds flavor and texture to the dish as it has been mixed with salt and starch from the pasta itself. The high concentration of salty starch helps to bind ingredients together, giving you a smooth sauce that's not overly runny. Additionally, using pasta water has the added benefit of providing just enough moisture that your finished dish won't be dry. Pasta water is also what you use to temper the eggs.