Product managers (a title from product companies) and product owners (a title from Agile teams) decide what features go into a product, and what those features look like. I am going to use the term "PO" or "product owner."
It's not important to build something. It's important to build the RIGHT thing. We know that this is a difficult problem because MOST software features are wrong. Users ignore them. You can do a lot of work and get no value. The product owner gives meaning to the work of everyone else on the team by making products that are meaningful to users.
Product owners are dealing with three big shifts:
1) Batch -> Continuous
The job used to be easier. In the old days, product owners had enough time to do their work because development was ... slow. They were often waiting for developers to finish the last batch of requests. However, developer productivity has increased dramatically from year to year and decade to decade. When you crank up your continuous Agile team, the situation will be completely reversed. Developers will be waiting for the PO. A full-speed continuous delivery development team can run over the product owners and leave feature roadkill.
2) Strategy -> Measurement
When you move to continuous delivery, strategy becomes less important, because you have more ways to get immediate feedback. For example, you can measure what users actually do. That is more accurate than what they say they want. Therefore the product owner has a responsibility to set up measurements, and after changes are released, loop back to analyze their impact.
3) Requirements -> User Experience
Your measurements will be strongly affected by usability. Usability, and the careful stewardship of scarce user attention, is the main battlefield on which most products are fighting now. You will need to understand usability in theory and practice, and measure it. You may find that requirements gathering becomes less important. A product can die if requirements get too far ahead of usability.