Although it is definitely true that they will learn after launch if the product is useful for its users, and if users understand how to use it, it is a very expensive way of learning and requires throwing away a lot of work when changes inevitably need to be made. Instead, user research offers a variety of methods to discover whether the concept of the product is useful, and if the implementation is going to be successful before launch, which is much cheaper, and quicker, than waiting until after launch.
