I think it's partially from an ethos driven with the hindsight of the original core Erlang team. They wished in retrospect that they'd spent more time document

I think it's partially from an ethos driven with the hindsight of the original core Erlang team. They wished in retrospect that they'd spent more time documenting "why" they did something rather than "what" they did. Not wanting to recommit sins of the past and all that :-)