Depending on whom you ask this question, the answer will vary. That is because in many companies the definition of product manager varies and has evolved along with the Organization.
But let me rephrase the question to: What should a product manager do??
That question now has a simple answer: The Product manager should decide what gets built as a product and what doesn’t.
But shouldn’t a product manager know UI/UX, technology and business? Yes, he should know all these things and keep learning to continuously to make sure the product turns out right.
Shouldn’t he be aligned with the vision of the founders, know the key success metrics and monitor and improve the product. Yes, he should. In determining what should be built he/she needs to factor in all these things.
But primarily the product manager should decide on what needs to be built and what shouldn’t be as the product.
Here is why:
Product managers are the voice of the users and represent users in every discussion when making decisions about the product. Doing this requires a deep understanding of the users , what their challenges and issues are, and how your product will deliver the convenience, value and of course the delight they are looking for. Good product managers always have their ear to the ground — usability testing, interviewing the users, working with the support team to listen to the customer issues or better directly work on the customer support team once a week. The best of the product managers synthesize this treasure trove of information into a clear and compelling direction for the product.
This is the essence of being geared towards a product market fit—being driven by the needs of the user and utilizing the capabilities of the company to deliver the product which delights the users and where the users themselves become the advocates for the product. A user-focued product defines itself by the users it wishes to serve rather than the features it wishes to sell or market.
Most of what a product manager does really just supports the essence that is described above.
Business case(the why of the product), talking to customers/users/management/legal/bizdev, writing MRD and PRDs, wireframing, working with UX design and even supporting the sales effort with the tools they need so that the customer can choose the right products and options.