IMHO
I have a DPI button on my mouse that I didn't have any use for. I got an idea that I could use it to control sound volume. I already have a ~500 lines of autohotkey script running on all my windows machines with a lot of custom hotkeys and I think this is the most universally useful part of it.
I mapped my DPI button to an F24 key so it wouldn't interfere with any app. The script converts vertical mouse movement to volume value and sets it with AutoHotkey's SoundSet function.
In my opinion it is super convenient even when mapping to some useless keyboard key.