to further clarify the power of qmk: you literally program (parts of the) firmware in c. so you can put modifiers everywhere you want, make layers until you are going crazy, make keys behave differently on press/hold, absolutely everything. and it's firmware you flash, so no matter where you plug it in, it' all there.
For the those new to the hobby and terminology, a layer is when you hold/lock one key (or a combination of keys) and then all of the other keys do something different than they did before.
The keyboard you're using right now probably has a second layer that is activated when you hold SHIFT. When you hold SHIFT, the letter keys will now be capitalized and the numbers will produce symbols.
Many programmable keyboards allow you to have more than two layers. For example, I remapped caps lock to activate a third layer, so now my home row and the nearby keys, can act like my arrow keys and common editing keys like home, end, delete, etc. I also mapped media keys to that layer, so "m" is mute and < > are prev/next song. Now I can do pretty much everything without my fingers leaving the home row. This is great for speed and fluidity as a programmer. I can also use a much smaller keyboard for better ergonomics and a more minimalist look.
I actually got into mechs for the ergonomics... I was having fatigue in my right (mouse hand) shoulder. Once I moved to a smaller keyboard (tenkeyless) and my shoulder wasn't turned as far out to use the mouse, my shoulder healed within a couple weeks.
Now I'm on a 60% keyboard. It took about two months to fully get used to my third layer, but I feel it was well worth it for the improved speed and flow.
qmk is just awesome, check it out https://github.com/qmk/qmk_firmware/