A PC-style keyboard isn't just an array of addressable switches. There's always been a microcontroller scanning the switches, disambiguating corner-case signals, and packaging it all into USB.

Probably the first driver for custom firmware is custom layouts.

Some people like some fairly exotic layouts. For example, a lot of smaller keyboards rely on multiple "layers" hidden behind modifier or toggle keys. Or you may just want a few programmable macros. For example, I have the incantations for my KVM switch mapped to a few uncommon key combinations.

If you're moving between different OS's or even machines, it might make more sense to bake all the smarts into the keyboard rather than to try to get several different software installs to behave consistently.

There are also a lot of gimmicky things you can add with custom features: per-key light effects, little OLED displays with graphics or statistics, faster scan rates.

Firmware is code, believe it or not.

You get some true horror stories coming from embedded engineers dabbling in software in this fields, the firmware is probably some of the least tested, verified, and generally visible code in many stacks.

I mean, even "regular" keyboards have source code these days, it's just usually closed and flashed to the board permanently. The days of keyboards literally being addressable switches ended decades ago.

But the reason why you'd want user programmable keyboards is the ability to change which keys do what. My keyboard has four different layouts ("layers") that have totally different keymappings. I use them for daily work, work macros, and video gaming.

Overlays, function keys, macro keys, fancy responsive RGB light shows, the list goes on.

> Why does a keyboard need source code

Because USB is very complicated. I'm sure you could do it 100% with hardware, but then your keyboard's source code will be Verilog since you would need an ASIC

To change the function of the fn keys. The keyboard is not just a dumb input, when you hold fn the board sends different codes for the same keys. This means you can’t easily move where the volume function keys are for example.

The HID protocol does not just implement it as addressable switches. In addition there's a question of which address each switch has.

Because it's the 21st century and every piece of hardware has some software in it... from your CPU to your keyboard.

There is more involved if you want USB, not to mention multi-keystroke macros and/or user configurable macros.

Even the dumbest and oldest PS/2 keyboard in your attic had some firmware blob on it. This is preferable.