This lets you split your interface into more isolated components, and makes you free to use different framework depending on the type of interaction your page needs, as well as be able to update your site progressively.
React/Redux hasn't been around long enough for its best practices to solidify in the general industry so you'll often find that companies that picked up React have a hodge-podge of incorrectly used technologies included because they didn't know the use case for Thunks vs Sagas, how to use Reselect correctly, whether to store any given state in Redux or a Container, and so on.
Also, in those cases a few extra seconds for the initial load or the general size of the application don't matter. It's all better than your average Swing application ("Challenge accepted", said the Electron developer).
Although this isn't something contemporary frameworks seem to aim for (ExtJS is dying, and that's good), you get close enough to desktop UI development with Vue/Vuex/Ant Design, Angular/AngularMaterial or React/Material-UI/Redux-boilerplate-reducer-of-the-week.