Contentful's UI Extensions feature allows developers to customize the web application with custom UI elements, enabling them to build interfaces that editors want to use and are adapted to their organization's needs. By uploading HTML files to Contentful and using the UI extensions SDK, developers can create rich interfaces without visual inconsistencies. The feature is particularly useful for building tables, references, and other complex content types that require specific interfaces. With UI Extensions, developers can improve the editorial workflow by simplifying tasks, reducing clicks, and providing a better user experience.