Just for demonstration purposes, and for total beginners in Javascript Web Apps to understand how it works.
Mind that I am by no means saying that SPA's should be done, or that they are done like this as this is only a simple demo. For the sake of developing real-world SPA's use a framework like Backbone, Angular.js, React.js, or one of the many great frameworks available. But it helps understanding what is sort of going on behind the scenes.
Enjoy.