You should reserve pasta water - here's why
If you’ve been draining your entire pot of pasta into a colander into the sink, you’ve been doing it wrong. You’re accidentally tossing out an ingredient that will take your pasta to the next level: the pasta water.
According to tests done by Serious Eats, adding starchy pasta water to a sauce will help emulsify it. Even if the sauce is just olive oil, the starchiest water in their test produced the most emulsified sauce.
An emulsion is a mixture of two or more liquids that are normally unmixable, like oil and water. They can be mixed by using an emulsifier like pasta water.
Many pastas (like cacio e pepe) require an emulsification for them to really work.
To learn more about emulsions in cooking, check out this website.