Answering the question “What is the most valuable item we can possibly work on” involves lots of research in the form of interviews, feedback breakdowns, experiments, user tests, behavior tracking, exception tracking, product metrics, etc.
Done right, this job cannot be done by product managers alone in the long run - even if they have very sophisticated technical skills. Therefore it has to be a team effort and that has an amazing side effect: When teams actively research customer needs then even the biggest tech nerds adopt a customer centric thinking. Also everyone knows exactly WHY they work on this or that feature. And that’s especially motivating when working on boring or really crappy tasks like fixing this ugly bug that only occurs in Firefox version X (but prevents 15% of your customers to use key features of your software).