The primary focus of user interface (UI) design in applications is to enhance user experience by creating visually appealing and intuitive interfaces. UI design involves incorporating user feedback and research to ensure that the application is easy to navigate and that the elements are logically organized. Key factors include functionality, accessibility, and aesthetics, all aimed at improving user satisfaction and engagement. Effective UI design plays a critical role in reducing errors and support costs while fostering loyalty and an enjoyable experience for users.