Customers Interviews not only tell you what to build, they save you big $$$ for usually $0. But they are also your first Marketing campaign. Interviews are actually a great way to spread the word about your product/service, but only if you can make the right approach. It's selling without selling.
In the 1970s several researchers looked into how different technology companies went from "unheard-of" to mainstream. What they discovered still holds true today, and is codified in Geoffrey A. Moore's book "Crossing the Chasm". Every product category follows this basic pattern of adoption, and you must learn to understand and master the 5 very-different types of customers to stay relevant in your market.