I've spent a lot of my time helping ours grow into a collaborating by walking them through my expectations of a product brief and what we need from our user stories. They're also really good at helping with the release, managing story sign-off and communicating changes with the other teams (customer success, account managers, sales team, etc). A good product manager manages... the product. They also collect all the user feedback, validate feature requests with data, and act as a guard for the rest of their engineering team.