For me it the epiphany was that I either want one full screen window, or several windows side by side, I never want one window to obscure another window. And with our modern wide monitors I almost always want several windows side by side. Why do I have to line them up by hand?
The downside is that there are many poorly programmed applications that are unable to adjust themselves as the window manager requires(calling you out steam, or it just may be me but steam was not happy with a tiling wm), and model dialogs, which always suck, suck even more when tiling. But most tiling wms's support a float mode for just that occasion.
Tiles were a common early metaphor, And I think tiles are generally more useful but floats looked cooler in the ad copy and so that is what ended up as the dominant desktop metaphor.
On the subject of one window partially obscuring another window the only tech that makes that halfway tolerable is "don't raise on click" and "point to focus" This lets you look at the upper window while manipulating the lower window. two features not found on the major desktop systems. X11 has it's warts, and is very weird. But it is miles ahead in actual usability over it's peers.
But on my desktop with two 27" monitors, I find tiling incredibly useful. It's really nice to be able to quickly arrange a mix of Emacs windows, browsers and random other programs to use all that space. If I need to closely read through some documentation while also looking at code, I can quickly open a browser next to the code window. Later, when I want to focus on just code, I can send the browser to the other monitor or to a different workspace altogether. And I can juggle "side" stuff (chat windows, OBS, music, whatever) just as easily.
The key thing is that XMonad with some lightly customized layouts gives me the operations to do all of this quickly without needing to think about it. It only takes a couple of keystrokes to get to the state that I want at any time and, after a pretty short time, those keystrokes became muscle memory. Using a mouse to drag things around is more intuitive and flexible, but it requires shifting my attention far more than just using a single keyboard command. (And thanks to ADHD, that kind of distraction can be a real drag on my mental energy!)
I don't use one, but interestingly, every IDE (or other professional mega-app such as Blender) is essentially its own tiling WM.
So quite clearly they have won as an UI concept for people who want to get things done.
And of that interesting stuff, how much can't be had just by configuring the window tiling shortcuts in floating window managers to have the best of both worlds?
I'm with you, that tiling vertically is not useful. Niri aligns better with my innate mental model: just keep adding new panes to the right offscreen that I can scroll through, then I can swipe up and down for more lanes to do collections of different tasks.
I feel much more productive doing read/write tasks, like viewing tickets + code or PDFs + tickets and so-on