In the end I just saw it as a strange and random language quirk. Like articles in German or Dutch.
I help out on /r/EnglishLearning, and a lot of the time my answers are like, "well, I don't know why but it sounds better this way...", which makes me think of the statistical model.
On the Spanish side, I recently thought about it with regard to the subjunctive mood, and in particular its use with the verbs pensar and creer (to think/believe). The upshot is that you use the indicative mood in the positive case, and subjunctive mood in the negative case (creo que estas bien / no creo que estés bien). People give lots of motivated reasons for why this is, and how in the negative case you're not "sure" of it, while in the positive case you're "sure" that you "think" it, or something like that. But ultimately, I think it's just the simple aforementioned positive/negative rule that native speakers build up over time from being exposed to sentence constructions, maybe without even realizing it, and only post-hoc rationalize why it is.
I don't see why this would be a "strange" and "random" language quirk when it makes perfect sense in practice. The only situations I can see where it wouldn't make perfect sense would be in situations where you're translating to English, which is not a productive way to think about languages.
It is 1 => Es la una (describing time uses Ser)
I am an engineer => Soy ingeniero (describing a profession which can be seen as a temporary state of affairs)
My favorite quirk is "estoy muerto" (I'm dead), which leads to plenty of metaphysical discussions.
Determining whether a sentence describes the subject or describes the subject's condition is the trickiest part.
"I am alive" uses "estar" -> "Estoy vivo."
But "I am dead" also uses "estar" -> "Estoy muerto."
AFAIK, when most people die, they are dead forever.
There are a few other ambiguous ser/estar situations we were taught, exceptions to the permanent/temporary distinction that I can't recall. But the near break-down this woman had about Paris was quite memorable.
And the longer I think about the word 'racja' the less I'm confident I'm able to explain precisely what it means using english.
I guess that's part of the fun of learning other languages, I will never understand articles in english and I made peace with it. Now I'm also learning spanish and subjuntivos drives me nuts but spanish and polish grammar are surprisingly similar (waaay closer than both are to english)