What are some things you are doing with this capability?

Like I said: "writing my WM config".

So, for example, I would write in my editor an expression to get the list of windows on the current display, then I can immediately map/filter through them, group, sort, read any parameters, etc. Then without missing a beat I can apply some transformations - programmatically move any window, resize, etc; on the fly, like playing a fucking video game.

Compare that to "more traditional" approach of issuing commands, then piping them to jq or something, then figuring out where the stuff is, then writing that into a file, then having your WM to pick up those changes, often you'd completely lose the state, then you'd have to go back to the script files, etc. I, don't have to save anything anywhere until I'm sure that shit works - I simply eval things on the go - I can "touch, grab and interact" with entities immediately after querying their info, there's no physical, mental or contextual separation here - I don't have to write something in the terminal, then something in one of my scripts, some parts in some other file, etc. - everything controlled directly from my editor.

Here's more practical example. Hyprland has something called Hyprsunset to deal with color temp and gamma. I wrote an extension that changes display color temp based on time of day - it's a simple clojure.core/async go loop, it reads from a hashmap where specific hours map to temperatures and then checks every 10 minutes; if it's time to apply new color, it does. That took me just a few minutes to whip out with a connected Lisp REPL. I'm pretty sure, it would've taken me far longer without it. The way how the "true" REPLs work (which e.g., Python one is not) simply is shockingly crazy awesome for rapid prototyping. Why more programmers don't do this is a complete mystery to me - getting into Lisp is not even that difficult, Clojure for example is far more simpler and more straightforward than even Javascript and Python. These days, you don't even need to know Emacs - install Calva for VSCode - that's all you need, it has quickstart guide and all.

Automated behaviours like forcing windows to different virtual desktops, modifying windowing behaviour dynamically. Some windows are automatically tiled and others float, etc.