Around 6.3V was a standard voltage for vacuum tube heater pins (the part that emitted free electrons via thermal emission inside the tube to be available for the electrical fields to accelerate).
My guess would be that historically 6.3V supplies made forvacumee tubes where commonly available when transistor based electronics where created so it made sense to utilize them. Works quite nicely for the typical 5V circuits as a "rough" input voltage to be feed through a LDO regulator, for example. And so it just stuck.
6.3V is the standard voltage of a farm tractor battery --these vacuum tube heaters were designed to use them.
12.6V is two farm tractor batteries (one car battery), which is why our computer industry uses 12V for motherboards (12 volts - 0.6V reverse protection diode).
Early computer power supplies used voltage regulators that were designed for car radios, originally.
What the hell is the point of having a hyper connected, super charged by AI world, when ultimately it boils down to having to "go to an office"?
I honestly feel like 95% of tech is smoke and mirrors, the people who own "tech" know this so they end up just asking people to come back to work to help them sell advertisements, which seems like the only way we know how to make money from all this "innovation".
It's really quite mad.