Fallback option for low end or deprecated devices
For almost all of us, me included, who are from a rich western country it's perfectly natural to have shiny new devices that can run all the latest and greatest browsers and apps. In many parts of the world it's not that simple though. Somethign like an old Firefox OS phone (or Windows phone, or Andoid 2.0 or whatever) may still be in use, and replacing it with a new Android device is just a huge investment that's not worth it "just" for Pleroma.
The FE needs a fallback to static HTML, otherwise we just keep to many people from using pleroma.