HTTPS, basically. I've gone around and around in circles on this for a device I work on. You'd like to present an HTTPS web UI, because a) you'd like encryption between the UI and the device, and b) browsers lock down a lot of APIs, sometimes arbitrarily, behind being in a 'secure context' (ironically, including the cryptography APIs!). But your device doesn't control it's IP address or hostname, and may not even have access to the internet, so there's no way for it to have a proper HTTPS certificate, and a self-signed certificate will create all kinds of scary warnings in the browser (which HTTP will not, ironically).
So manufacturers create all kinds of crazy workarounds, like plex's, to be able to present an HTTPS web page that is easily accessible and can just talk to the device. (Except it's still not that simple, because you can't easily make an HTTP request from an HTTPS context, so plex also jumps through a bunch of hoops to co-ordinate some HTTPS certificate for the local device, which requires an internet connection).
It's a complete mess, and browsers really seem to be keen on blocking any 'let HTTPS work for local devices' solution, even if it were just a simple upgrade to the status quo that would otherwise just be treated like HTTP. Nor will they stop putting useful APIs behind a 'secure context' like an HTTPS certificate implies any level of trust except that a page is associated with a given domain name.
(Someone at plex seems to have finally gotten through to some of the devs at Chrome, and AFAIK there is now a somewhat reasonable flow that would allow e.g. a progressive webapp to request access to a local device and communicate with it without an HTTPS certificate, which is something, but still way to just host the damn UI on the device without limiting the functionality! And it's chrome-only, maybe still in preview? Haven't gotten around to trying to implement it yet)
See this long, painful, multi-year discussion on the topic: https://github.com/WICG/private-network-access/issues/23