Well, the idea is that you render the first pass of the SPA and serve that HTML to the client. Rather than just injecting the data into the SPA source code, you inject the data and render the page, and then serve that page. Then whatever changes the user makes to the page after that initial load gets handled by the SPA. This way the user avoids having to wait for the SPA and all its dependencies to load before the page can even appear, and the developer doesn't have to differentiate between what happens on the client side and server side (remember using PHP to inject variables into JS source code?) because they can just bake it into their SPA.