The big advantage of a library like React is that you can specify your UI declaratively once, instead of having to specify imperatively how to handle every poss

The big advantage of a library like React is that you can specify your UI declaratively once, instead of having to specify imperatively how to handle every possible state transition. This can dramatically reduce the number of different rendering cases you have to write if you have a lot of state to manage. All the virtual DOM stuff is just an implementation detail that makes the declarative stuff fast enough to use in production. A more naive implementation that rebuilt your entire (real) DOM on every state change would not be fast enough for anything but the most simple applications, because DOM updates in browsers are currently a relatively slow operation. Browsers could provide an API to more efficiently update large sections of the DOM at once, using a React-style diff algorithm or otherwise, but currently that isn't a feature they offer. If they did, you'd be correct that there would be less need for libraries like React.