A PC-style keyboard isn't just an array of addressable switches. There's always been a microcontroller scanning the switches, disambiguating corner-case signal

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.