I still use that feature of X Windows, probably its BEST feature, to run X11 programs on headless (and even the SAME) computers.
If you do export DISPLAY=localhost:0.0 and enable TCP and 'xhost +localhost', you can THEN su to whatever user you want, and run X11 applications under a different user context.
SO I can be logged in as 2 or 3 or more users on the same X11 desktop, much more easily than "run as": under windows.
And then there's development on an something like an RPi. Use 'pluma' to edit your code directly on the RPi, and *NOT* have to struggle with a tiny touch screen or separate HDMI-capable display? I do that a LOT, especially with 'headless' RPis.
But I think that this is most useful for web browsers. If you configure Firefox (for a particular user) to automatically purge ALL history, cookies, data, etc. every time you close it, you NOW have a stealthy browser that has NO history that could even POSSIBLY be abused by ANYONE.
And you can run that browser in the security context of a user that doesn't matter. "Oh, that downloaded thingy just wiped out my home directory. Oh, well, so what. *yawn* [rebuild] no problem now"
So yeah, the SINGLE BEST FEATURE of X11 is its inherent remote client/server configuration.
(and Wayland cannot do that, nya nya-nya nya-nyaaaa nyaaaaa)
20 years ago i was doing stuff with windows boxen. 15 years ago I decided that it was DEFINITELY worth doing a lateral to POSIX systems like FreeBSD and Linux, as it appeared that Windows was just simply going the WRONG direction in 2003 (and I know I was right).
Guess what I stuck with? yep!
30 years ago I was doing stuff with PCs, combining a process involving Lotus 123, Harvard graphics, Word Perfect, and a bunch of '.bat' files, minicomputer report scripts, downloading, and overall "process automation" to generate a multi-page weekly report on Monday AM that reflected the most up to date data and presented it in a manager friendly way. TOok about 2-3 hours to print so I started it at 6AM, showed up around 8 AM, made sure the paper didn't jam [or Id hav to re-run it], got it all copied and stapled by 9 AM, had it all delivered by 10 AM, and then said "Wow, my week is done. Now what do I do? I think I'll work on THIS today..." [and they let me - that report took a week to generate before I started, had less stuff in it, didn't look as nice, and was 4 days old data-wise when it was finally completed]
NOTE: because I don't like doing data entry or futzing around with presentation on paper, I came up with a way for the computer to DO ALL THAT WORK FOR ME because I'm LAZY and I _HATE_ _DRUDGERY_. Worked out pretty nicely for all parties involved.