React is a popular JavaScript library for building user interfaces. It offers a number of benefits that make it a valuable tool for web development. Using React provides several advantages over building a UI with vanilla JavaScript and HTML:

1. Component Reusability: React allows us to break down our UI into reusable components. This modular approach makes it easier to maintain and update our codebase. We can create components once and reuse them throughout your application, reducing redundancy and promoting code efficiency. This reusability can save development time and effort.
2. Virtual DOM and Efficient Rendering: React uses a virtual DOM to optimize rendering performance. When changes occur in React components, React updates the virtual DOM first and then efficiently updates only the necessary parts of the actual DOM. This process minimizes expensive direct manipulations of the real DOM, resulting in faster rendering and improved overall performance.
3. Declarative Syntax: React's declarative syntax allows us to describe how the UI should look based on the current application state. Instead of imperatively manipulating the DOM with vanilla JavaScript, we declare what the UI should be and let React handle the underlying updates. This declarative approach makes our code more readable, easier to reason about, and less prone to bugs.
4. State Management: React provides a straightforward way to manage application state. It encourages the use of component-level state and allows us to easily pass data between components using props. React's unidirectional data flow ensures that data changes propagate predictably, making it easier to understand and debug your application's state.
5. Ecosystem and Community Support: React has a vast ecosystem with a wide range of libraries, tools, and extensions that enhance its functionality. These resources can help with tasks like routing, state management, form handling, testing, and more. Additionally, React has a large and active community that provides ample support, documentation, and community-driven solutions to common problems.
6. Developer Productivity: React's component-based architecture and developer-friendly features can significantly improve productivity. The ability to reuse components, the availability of pre-built libraries and tools, and the ecosystem's support all contribute to faster development cycles and easier maintenance.

While it's possible to build UIs using vanilla JavaScript and HTML, React simplifies the development process, enhances performance, promotes code reusability, and provides a robust ecosystem. Ultimately, the choice between using vanilla JS or React depends on factors such as project complexity, scalability needs, developer familiarity, and the specific requirements of the application.