Pasta water is useful because it brings both liquid and some starch. It can help a sauce coat the pasta instead of leaving oil at the bottom of the bowl.
Before draining, scoop out a mugful with a heat-safe vessel. Add a little to the sauce as you toss the pasta. Keep stirring and watching: you want a coating, not a pool. Different pasta shapes and cooking methods produce different water, so a fixed volume is less helpful than the result in front of you.
Remember the salt. If the cooking water is strongly seasoned, reducing large amounts into a sauce can make dinner too salty. Use plain hot water when that is the better choice. Serve promptly, because pasta continues absorbing liquid after the heat is off.