This site is entirely contained within a single file, with the exception of the binary image data.
There are a number of reasons why one would want to take this approach with a website, particularly statically-generated sites. The biggest and most important reason is efficiency.
By reducing the total number of connections made by the client browser, we reduce overhead. Webservers can be configured to utilize GZIP compression, web application firewalls can be configured for aggressive caching and so on.
There are many situations of course where the single-page-all-content model breaks down and that's when you've got dynamic or user generated content or there is just so much content that it makes downloading all of it not worthwhile when the average visitor only visits a handful of links.
Always use the right tool for the right job and this is just a simple example worth knowing.