[bosdev]

To me the logic is simple: It's more efficient to build the site when its content changes (very infrequently) rather than when its viewed (very frequently). Yes various forms of caching can allow you to scale the dynamic model quite far, but scaling gets so much simpler when you move into the static world.

In answer to your questions, the general idea is you have a static site, and a powerful API. The power of the JavaScript engine in modern browsers allows you to build your entire application to run in the browser, sending asynchronous requests to your API as is required. You're essentially just removing the Python/PHP/etc. middleware layer many sites have traditionally had.