One of the key responsibilities of product manager is to make sure that product is usable by the customers. This is a difficult task for a novice product manager. That's because it is easy to jump right into designing product without understanding what the problem is to be solved and how the product should help the user solve the problem.
Often this understanding the problem and testing the solution step receive cursory review. This shows up later in the product training and deployment phase, and by the time it is recognized it's often expensive to change the product behavior or user interface.
That's why all product managers should thinking about building a prototype. This is especially true for a Version 1.0 product because the risk of missing the key value to customer is far greater because of initial engineering investment cost is higher hence product managers have to make lots of trade-offs. It's easy to trade off something critical and only to release a product that is not good enough for anyone.
