The primary function of a user interface prototype is to visualize and test the design and functionality of an application before full-scale development. Prototyping enables designers and stakeholders to explore different layouts, features, and user interactions to ensure the final product meets user needs. It helps identify usability issues early in the design process, allowing for necessary adjustments based on feedback. Using tools like Figma or Adobe XD, creators can develop interactive prototypes that facilitate user testing and drive more effective development outcomes.