These UI Frameworks enable us to create single-page applications where most of the heavy lifting is done on the client. Traditionally, the web server has been responsible for routing, generating UI from templates and querying backend system. In modern SPA applications, these tasks are performed on the client inside the browser. These frameworks have grown in popularity as they enable us to manage all this complexity on the client-side. In particular, they help us define user interfaces in a declarative format which keep themselves updated as the state of the application changes.
To keep the UI in sync with application state, React and Vue use virtual DOM technology. It minimizes the changes to the browser DOM by avoiding the recreation of large portions of DOM with every state change. The idea is to keep a duplicate copy of DOM in memory (called virtual DOM). As the application state changes, rather than applying the changes to actual DOM, they are applied to the virtual DOM. Virtual DOM is then compared with the actual to find the differences. Finally, the differences found between the virtual and the actual are applied to the DOM. This helps in minimizing the amount of changes to the UI.
Svelte syntax extensions are also very intuitive and easy to pick up. It has a much smaller learning curve then Angular or React. And has a lot less boilerplate code.
Svelte is much younger, though. There is much less community support and third-party libraries as compared to the other three frameworks.