That's conflating marketing and advertising when they're actually two different things. Think of marketing as the social component of design.

You wouldn't start coding without at least a rudimentary spec to guide what it is you want to make, right? So why start designing your product without first identifying what and for whom it is you are designing it?

Marketing is the process of identifying the target market for your product. The "what it does" and "who it's for" part that a design needs if you're going to "build something people want".