The JSON metadata files are published once a day, in parallel with the XML metadata. First we publish the signed XML (~3:00pm) and then we convert the XML to JSON (~4:00pm). Being unsigned, the JSON files are meant exclusively for information pages (like the Federation Info Pages), not secure web applications.
This architecture has distinct advantages over other web application architectures. First, the runtime environment of the client-side applications is completely independent of all other infrastructure, most importantly, all backend databases. Second, since the page content resides in the JSON files (not the HTML), there is a distinct separation of presentation and content not achieved in other web development frameworks. These advantages make these applications easy to create, modify, and maintain.