When designing a new application or making revisions to an existing application, there is no substitute for the direct experience that end users have. An end user can give immediate feedback on your design to point out awkward or incorrect functionality. They also help to ensure that all controls are logically placed for the most efficient use of the system.
Using an interactive prototyping tool allows you to walk a user through the interface or even allow them to work directly with the prototype so they can quickly suggest improvements. As use cases are being developed, it is a good idea to walk users through the use case to ensure that the use case is well thought out and that all functionality is captured both in the use case and the prototype.