Define PM in three words: Predictable Quality Delivery.
I can't help to look at PM from the Product Management angle rather than from the Project Management angle. The three constraints (scope, schedule and cost) might be great for building the first version of a product but enhancement, maintenance, the future is a different story. Without a constant pace delivery it will be difficult to remain competitive. That constant pace cannot be supported if quality is not the number one concern in your production lane.
In order to provide value, PM should ensure the team has "hight quality predictable delivery".