React is often used for building single page applications (SPAs). SPAs tend to have multiple page views. When navigating from one-page view to another, reloading the entire page view is a tedious and not so efficient task. In fact, it diminishes the benefits of a SPA. To work as it should, a SPA must render parts of views when required instead of reloading the entire page.
This is a companion discussion topic for the original entry at https://blog.crowdbotics.com/introduction-to-react-router-v4-with-reactjs/