Here are the top ways I'm using it right now:
1) Hide/show apps similar to how iTerm lets you bind a hotkey to hide/show a terminal. I've got ctrl+space set to Vimcal, alt+space set to midnight.app (a time tracker I'm building), and ctrl+alt+space set to Things.
2) Start/stop playing my work playlist of lofi hiphop. I got tired of the friction around opening Spotify, going into my work playlist, hitting play, waiting several moments for the playlist to load, etc, so I downloaded a bunch of mp3s from YouTube and put them in ~/Music/work/. Now my work music is a single keyboard shortcut (semicolon+m) and a few miliseconds away.
3) Set up screen recording. There are a few things that need to happen before I start a screen recording (opening CamHead.app, setting my screen resolution, and showing the dock at a certain height so I can later crop the video to 16:9) and I have it all bound to a single hotkey (semicolon+r). I've got another hotkey set up to unset it (semicolon+e).
4) You may have noticed I'm using keyboard shortcuts with semicolon as a modifier key. That's done with Hammerspoon too! I've got an extra layer of hotkeys available to me to set up whatever else I can think of in the future. The code I wrote to use semicolon like this sometimes breaks if I try to type too many actual semicolons in a row but I usually rely on JS Beautify to add those for me.