Understanding the Applications of React in User Interface Design

React is a prominent JavaScript library that aims to simplify the process of building user interfaces. Developed by Facebook, React is widely adopted in developing both single-page applications and mobile apps. It allows developers to create reusable user interface components, which not only improves code efficiency but also enhances performance and maintainability.

React's emphasis is mainly on providing seamless user experience through efficient rendering of user interfaces. For a practical example, consider a news feed like Facebook's or Twitter's. With data regularly being updated, traditional methods can result in rendering delays and a less fluid user experience. However, by leveraging React's virtual DOM, updates can be processed in a more efficient way, leading to a smoother and more responsive user interface.

Moreover, React utilizes a component-based architecture. This means that the interfaces are built out of components, each managing their own state and rendering logic. As a result, this encourages reusable code, increased maintainability and scalability.

Applying best practices in using React for user interface design includes the proper use of states and props, life-cycle methods, and key principles such as unidirectional data flow. Also, to make projects more manageable and scalable, developers often use additional tools like Redux and React Router that better handle state management and routing, respectively.

In conclusion, React is utilized primarily for building user interfaces due to its efficiency, reusability and component-based architecture. Its emphasis on providing a seamless user experience has widely popularized its use in today's web application development landscape, hence the correct answer - React is mainly used for building User Interfaces.