I can't even work on Mac without it. It let's you do stuff like "alt+spc a b" (apps -> browser) or "alt+spc m j/k" (media -> vol up/down), or edit just about any text of any app in your editor (Emacs atm) - with all the tools you have there - spellchecking, thesaurus, translation, LLMs, etc.
You can plug it to your favorite WM (I'm currently using Yabai) and do tons of other interesting things. Because it's all written in Fennel, one can develop things in a tight feedback loop with a connected REPL - e.g., I can ask Claude to inspect things in the running Slack app or Firefox and make interesting automations - all without ever leaving my editor.