Comment by gunapologist99

REST API's (literally, "Representational State Transfer", ReST) are a very nice match with SPA's, because now the browser is the sole source of truth for application state (and the server only has to produce some JSON data instead of entire HTML pages).

Literally the entire point of REST is that you don't have to maintain server-side sessions, and you shouldn't. This also has benefits for scalable applications, in that you can transfer your running application to a cluster of servers on the other side of the world, and it'll still keep working.

The trust boundary between the front and back ends can be a huge boon to security as well, and some things are literally impossible to do with a multi-page app, and you can still maintain URL parity with new actions in your page.

That's why SPAs were, and are, great!

That doesn't mean everything should be an SPA! A photo, news, blog, or recipe website might be better laid out as a separate page for each individual item. However, there are tons of other types of real applications that can only be built as an SPA.

Are they unwieldy, sometimes hard-to-code and maintain, and sometimes result in a poorer experience? Yes, but that can also be true of a multi-page app, especially if you're trying to shoehorn an SPA experience into a multi-page app and want to maintain the same user experience as a user moves around within the app.

A multi-page "app" isn't really an app anymore; it's a server-side app that is producing multiple pages. A single page app literally is running in the browser and only sending a bit of data back and forth with each user interaction. It's just a completely different model.