It's primarily useful to people who already use emacs a lot, I think. Not sure I'd recommend using emacs just for EXWM, as some people seem to do with magit or org-mode.

The main problem it solves is that emacs is a tiling window manager (in a loose, non X-specific sense), so if you spend the majority of your time in emacs it gets quite annoying that you're having to use two different paradigms to manage windows: one for managing emacs "windows" and the "buffers" that they "visit" and one for managing X windows, of which emacs itself is one. EXWM makes X windows behave (mostly) as if they are emacs buffers, so you can use the same powerful tools to manage them.

As an example, I use ivy (https://github.com/abo-abo/swiper) so for switching between buffers I get a nice incremental fuzzy-matched buffer list. With EXWM that same buffer list also contains my web browser and other X applications.

I've written a toy window manager so I have some sense for what making a real one would entail. I've been using emacs for 20 years or so, so have some insight into that. I am in awe at the dedication and attention to detail it must have taken to turn EXWM from a crazy idea into a solid WM that is completely usable as a daily driver.