There’s typically a lot of uncertainty in requirements and it is unrealistic in some cases to expect users to articulate detailed requirements upfront without even seeing the end product. This is a fatal flaw in the Waterfall model and this is a key reason why an Agile approach has been so successful. Agile does not rely on defining detailed requirements upfront prior to the start of the project. Instead it uses an incremental and iterative approach to development where the requirements are further defined and elaborated as the project is in progress.