User research and testing play a vital role in UI and UX design. Without user research, designers would be working in a vacuum, without any feedback from the people who will actually be using the product. And without testing, there would be no way to know for sure whether the design is effective and usable.
User research helps designers to understand the needs and goals of their users, and to identify any potential problems with the design. Testing allows designers to see how users interact with the product, and to make sure that it is easy to use and effective.
User research and testing are essential to UI and UX design, and should be given careful consideration at every stage of the design process.