Before a team can begin building product, someone must go out and discover what needs to be built. A Product Manager is tasked with finding the most important problems and defining which of them should be solved.
The best way to deal with things that can change is to develop the minimum of what you need right now instead of speculating on whatever you may need later.
Incremental Delivery is the idea of looking at different ways to solve a problem. It aims to deliver the big project in small increments instead of aiming straight at the final big thing.