The end user’s goals should be the goals of the client and their product. It is essential for any UI system to help achieve those goals in as easy and efficient a manner as possible. They should be a guiding principle in all phases of a project, re-affirming these goals often. So it is essential to learn as much as possible about the users’ skills and experience, and what they need. Discover how they interact with the current User Interface and watch how they use it. Understand why your users make the choices they make, and not just which choices they prefer.
The more you understand your end users, the easier it becomes to design and develop a User Interface system that leads to an enjoyable and beneficial user experience.