I feel that it empowers me as a Backend developer and makes me productive on both the BE and the FE.
For one it adds a simple set of primitives I need to learn on top of HTML (a basic tech to know), so no large JS frameworks or patterns to learn.
Secondly, it's really fast with regards to development speed, application speed and just getting up to speed in general. A basic MVC framework (such as Django but others also) is all you need in addition to the primitives I talked about above. With modern frameworks such as Vue or React, I need a backend with basically the Model and Controller part and then a different framework for the View part.
If you structure your backend correctly, it should be trivial to build a JSON API on top of your service layer. Unless you're coupling your http endpoints to your service logic.
I'm a Front End Dev and work with SPAs for the past 2 years. Working an offline-enabled PWA powered by Svelte and Redux. Honestly, I have a hunch that an HTML-based oriented development can still provide a great (measured) offline experience, without needing to rebuild wholesale to SPA.
It's a totally different ballgame if the app is mostly client-side utility anyway (calculators, etc), but the HTML + a bit of JS handling Service Worker and offline presentation should enable a great offline experience. I haven't seen it done in a large scale, but it should work.
It's not exactly a lot of work to handle HTML for web apps and have an API for mobile clients or services. For example, if you are using Django, just add DRF with some serializers etc and re-use any common backend logic with both frontends.
For one it adds a simple set of primitives I need to learn on top of HTML (a basic tech to know), so no large JS frameworks or patterns to learn.
Secondly, it's really fast with regards to development speed, application speed and just getting up to speed in general. A basic MVC framework (such as Django but others also) is all you need in addition to the primitives I talked about above. With modern frameworks such as Vue or React, I need a backend with basically the Model and Controller part and then a different framework for the View part.