e.g. pressing a shortcut like "Ctrl + b" most likely involves stretching your pinky finger on a standard keyboard. One feature custom keyboards often have is dual-use keys; keys which act the same if tapped, but have a different function if held. So e.g. tapping 'f' outputs 'f', holding 'f' is the same as holding 'shift'. With that technique, you can put Shift, Ctrl, GUI, Alt all on the home row.
Custom firmware can also support chords; e.g. pressing 'cv' at the same time could be a chord to switch to the previous virtual desktop.
Custom firmware also allows layers (similar to how some laptops have a 'numpad' in the middle of the keyboard which can be used if Fn is held). This allows more flexibility, at the cost of more complexity.
Much of this could be done with e.g. kmonad, but it's nice to have it on the keyboard itself.
All my modifier keys are on the home row. E.g. F is Shift until I release it, or F after I release it. My pinkies are not overburdened, and I'm an Emacs user.
Then, layers. I don't need to move my hand to the numpad; I press a modifier with my left hand, and the numpad is instantly under my right hand. Same with shifted symbols like @ or &; I can have them right under my left hand, without reaching far. Same for media control keys.
QMK also emulates a mouse, so I can move the mouse cursor, click, scroll with keys, pressing another modifier.
Also you can assign macros to keys. I have little use for it, but people playing games, or people using certain unergonomic software, appreciate it a lot.
This all is, of course, fully configurable.
If I'm working in a DAW such as Pro Tools or Logic, I can do all sorts of things such as program keys to play full chords or chord progressions. Maybe I want to press some key combo to start/stop my external sequencer, or arm/disarm a synth to improvise live with a looper, but not record into the loop.
I'm sure the possibilities are endless, but having the flexibility to tinker and build your own workflow is super neat.
Plug your keyboard into a new computer and the keymapping stays the same.
That said, I have forgotten what the third layer shortcuts are, because I never actually used them after day 1.
[0] !https://beta.docs.qmk.fm/using-qmk/software-features/feature...