everything from rearranging the keys (for instance I use vim, so I swap esc and capslock), to changing tap and hold behaviors (my capslock key does esc on tap and ctrl on hold), to really elaborate stuff like tap dances (thomasbaart.nl/2018/12/13/qmk-basics-tap-dance/), all while being configured directly on the firmware of the keyboard, so it just works no matter what computer it’s plugged into, and it doesn’t have any weirdness that comes with software remapping tools.