React.js, often referred to as React, is a popular JavaScript library for building user interfaces (UIs) for web applications. Here are some reasons why you might choose to use React.js for web development:

1. **Component-Based Architecture**: React.js follows a component-based architecture, allowing you to break down your UI into reusable components. This modular approach makes it easier to build and maintain complex UIs, as each component encapsulates its own logic, styling, and behavior.

2. **Virtual DOM (Document Object Model)**: React.js uses a virtual DOM to efficiently update the UI. Instead of directly manipulating the browser's DOM, React updates a lightweight, in-memory representation of the DOM. This results in faster rendering and improved performance, especially for large and dynamic web applications.

3. **Declarative Syntax**: React.js uses a declarative syntax to describe how the UI should look based on the application's state. This makes it easier to understand and reason about your code, as you can focus on what the UI should look like at any given time, rather than the imperative steps needed to update it.

4. **JSX (JavaScript XML)**: React.js uses JSX, an extension of JavaScript that allows you to write HTML-like syntax directly within your JavaScript code. This simplifies the process of creating UI components, as you can write markup and logic in the same file, improving readability and maintainability.

5. **React Hooks**: React introduced hooks in version 16.8, allowing developers to use state and other React features without writing class components. Hooks, such as useState and useEffect, provide a more functional approach to managing state and side effects, making it easier to write and test React code.

6. **Community and Ecosystem**: React.js has a large and active community of developers, with a vast ecosystem of libraries, tools, and resources available to support development. This includes third-party libraries for state management (e.g., Redux, MobX), routing (e.g., React Router), and UI components (e.g., Material-UI, Ant Design), among others.

7. **Server-Side Rendering (SSR) and Static Site Generation (SSG)**: React.js supports server-side rendering (SSR) and static site generation (SSG) out of the box, allowing you to render React components on the server and generate static HTML files at build time. This can improve performance, SEO, and accessibility for your web applications.

8. **Backed by Facebook**: React.js is maintained by Facebook and used in production by several of its products, including Facebook, Instagram, WhatsApp, and Messenger. This backing provides confidence in the stability, reliability, and long-term support of the library.

Overall, React.js offers a powerful and flexible framework for building modern, interactive web applications, with a focus on performance, scalability, and developer experience. Whether you're building a simple website or a complex web application, React.js provides the tools and capabilities you need to create engaging user experiences on the web.