Grammar is the way we structure our language.I lived through 2 intellectual and almost spiritual ectasy in my life...
The discovery of the distribution of prime numbers...But this is another story...
And the deep meaning behind ANY "apparently arbitrary" rule of grammar...
People who think that grammar of any language is arbitrary are deluded or ignorant of linguistic...Grammars mirror completely the neuro activity of our brain in a different mode of expression in each culture...
i cannot vouch too much for Gustave Guillaume psychomechanic of language and discourse.... Alas! only in french....
Grammar is boring if we stay at his surface level, in his deep internal mechanism it is like a LSD trip in a higher mathematic....
It takes a genius to guide us.... Guillaume is certainly one in more than 40 books.... If you read french try it....
For me there is 2 levels in language dynamic genesis:
The deep motivation of sound-speech on one unconscious level on a long time scale in relation with words construction and formation and the " feeling" aspect of human constitution ..
And a corresponding relative demotivation of the sound speech which correspond to what we describe generally as sentence "grammar" in a more conscious level corresponding to the " thinking" aspect of human constitution...
The 2 opposite mechanisms work together on 2 levels...This idea is mine, not strictly in Guillaume like i expose it here... But it gives the flavor of my reflection...
These 2 levels could be resumed in a simplest way if you remark that they match and correspond to poetic mode working and the prosaic mode working of language....
Now for the REASON to be REASON these 2 working mode are necessary if not, human degenerate into irrational animal or so called "rational" machine....
Then to be "human" is being able to create and accomodate together all derivative modes of these 2 archetypal symbolic matrix and modes : the poetic and the prosaic in language...