Frameworks like React and Angular assist in building web GUIs in several ways:

1. Component-based architecture: Both frameworks encourage building reusable UI components, making it easier to manage complex interfaces.

2. Templating and rendering: They provide templating engines and rendering mechanisms to efficiently update the UI.

3. Data binding: Automatic data binding simplifies the process of connecting UI components to data models.

4. State management: Built-in state management tools help manage application state and props.

5. Event handling: They provide standardized event handling mechanisms for user interactions.

6. Dependency injection: Dependency injection systems simplify the process of managing dependencies between components.

7. Routing and navigation: Built-in routing mechanisms facilitate client-side routing and navigation.

8. Modularization: Both frameworks encourage modular code organization, making it easier to maintain and scale applications.

9. Community and ecosystem: Large communities and ecosystems provide extensive libraries, tools, and resources.

10. Performance optimization: Built-in optimization techniques, like virtual DOM (React) and change detection (Angular), improve application performance.

By leveraging these features, developers can build complex web GUIs more efficiently, with improved maintainability, scalability, and performance.