Why React example does not use Async call to fetch json of requested page

I am following this tutorials: https://github.com/adobe/aem-sample-we-retail-journal

Here, all the child pages JSON is stored into __INITIAL_STATE__ id. 

I want to know why it is done in this way, can someone please explain?

Instead what if I navigate on different links and then on click of each links if I call model json of that page via Async fetch and then accordingly set __INITIAL_STATE__ id. What problem in this approach?