[pstachio]

I work as a product manager and hardware engineer. In my experience the fundamental role of a successful product manager is to understand the users of the product. And this isn't guess work, its based on hundreds of hours of interviews and interactions with users and potential users, its figuring out ways to prototype, test, and refine features before committing to build, its being able to articulate how the product meets and exceeds user needs. It means that when feature prioritization pops up during sprint planning, the product manager can talk authoritatively about what the user wants or needs. And a good product manager will have involved the dev team in some of these user interactions so the devs can empathize with the users as well.

This is not a role fulfilled by developers or scrum masters or project managers. If you want to repeatedly make great product, instead of just the lucky guess / shot in the dark that a lot of startups rely on, you need someone whose whole job is deeply understanding the user and making sure the product under development will meet and exceed the user's needs.