Here's what she had to say!
"[Starchy pasta cooking water] is an ingredient in every sauce," Rach explains. "It marries the sauce to the pasta."
"You want all of the pasta to taste like all of the sauce," she continues. "So whether you're marrying pesto, red sauce or a white-based sauce, you need that starchy liquid to get everybody to get along and come together."
Plus, "this is why we salt our water before we cook pasta," she says.
Pro Pasta Tip: Always undercook your pasta by 2 minutes — or at least 1 minute — because you're supposed to be tossing the pasta and sauce altogether, allowing for carryover cooking. So, drain your pasta 1 to 2 minutes before what the package suggests for al dente pasta, Rach suggests.