One of the reasons why we have front end apps pulling
    from an API is because it allows for interoperability.

    The same API that serves data to the web browser can 
    serve that data to mobile apps and to third parties as well.

I agree with the benefit of API interoperability, but I'd call that orthogonal to server-side rendering. Your server-side web app could be just another consumer of that repurposable API.

    The idea of bringing HTML rendering back on to the server 
    just doesn't seem useful to me. 

It's a streamlined developer workflow, for one. You have one renderer, not an infinite amount of client-side renderers. Easier one-stop-shopping debugging. And in reality, I feel that the promised performance gains of client-side rendering never truly materialized for many use cases.

For projects involving small teams and non-trivial amounts, this is a boon. Any backend dev can, at least, write code to spit out some semantically correct HTML. Somebody is going to need to work on the UI/UX at some point but not necessarily full-time.

Contrast with a client-side rendering approach. If there is any significant amount of backend engineering behind the site (there isn't always, of course) now you have two apps with two divergent skillsets and more often than not, you need a minimum of two developers.

That's if you're making an application, sure, but on the web I view all sorts of sites and most of them are just styled text containers, maybe with an optional way to mutate things. HN, Reddit, News, recipes, forums, galleries, these are things I regularly consume that don't need APIs to return content. For applications, like Facebook, it's understandable, but if you don't spend all of your time in Social Media, then you probably are not really using an application.

You must use old.reddit.com

On mobile I just use a client that requests data from the API and renders it for me. On desktop, yes, for now, but I'm in the process of writing views that load faster and don't require a browser.

Maybe I'm being dumb (I'm a sysadmin, not a developer) but wouldn't you just shift those API calls to the server rendering the page rather than the browser?

Yes

Having both is still an option with minimal effort. In Rails it can be as simple as adding .json to your request and you get the data, otherwise the HTML.

You're correct, but for many companies, APIs and multiple interoperable clients are a YAGNI.

You mean the locked down apis that are not available to the puclic? Those can rot in hell.