They don’t send a keycode, they modify existing keys so they either send different keycodes or modify the keyboard’s state (e.g. toggle LEDs).

The main difference is, once you start rabbit-holing the custom layers, you don’t have to only put a layer on the F keys.

My favourite layer is holding down an extra thumb key to get arrow keys + home/end, pgup/pgdown near home row.

Since I have yet an extra thumb key, I made it so I get those navigation keys but holding down Ctrl, so I don’t need to stretch across both a Fn key and a modifier. And so on.

I have a combined Tab + Ctrl key: Ctrl on hold, Tab on click. All of these just work, no adaptation, no downside, just less movement.

I’ll never go back from this.