WHY each NT author wrote the book is because they were guided by the Holy Spirit to do so, as well as WHAT they put into it.
Men are errant - so, if the author is hot the Holy Spirit, through the person of the scribe (writer of the NT, or OT scripture) then the whole book can be errant.
In fact, Paul says in no less than 3 places (1 Cor 7:12, 1 Cor 7:25, 1 Cor 11:17) when he is writing according to Paul’s will versus the Holy Spirit leading him.
So, you CAN say there are 3 versus in the NT that are not God-Breathed. Paul tells us which ones HE wrote.
I’m sorry, but your premise, if I’m understanding it correctly, doesn’t make any sense. Your proposing that Spirit-inspired scripture, by definition, has no historical reason for being written? Why can’t it be both?
Can’t scripture be God-inspired, AND have a historical context? Studying the scripture strongly indicates Luke wrote Acts, an orderly account of the past 30 years of Christian history, to Theophilus, a Roman official, with the express purpose of legitimizing Christianity to the Roman government in the face of the impending Neronic persecution.
But how does that contradict divine inspiration? How do you know the Holy Spirit didn’t move on Theophilus to request the acccount, then whisper what to write to Luke?