Product is a tool used to address a market opportunity. Idea of a product is conceived from the opportunity, and it (product) takes shape as it grows and as needs evolve. Keeping track of evolving needs and growing product is a function of a product manager. Product Manager makes sure that they keep track of the evolving market by uninterrupted communication with stake holders, customers, engineering and market. These needs are consolidated, filtered and refined to define product road-map and to draft requirements of every product release.
It is critical that a product manager understands the difference between managing product verses managing opportunity. A product manager should always look at product from the lenses of the opportunity. How well does the product fit into the opportunity? , what are the improvements required? , Is my product aging and should I start looking at alternates? Which features would make product a better fit for the opportunity? Looking through the lenses of the opportunity, if a product manager sees a clear and sharp picture of the product then this justifies the very existence of the product.