"Why it was there" is something that matters to people, plural. Knowing why it was for some people doesn't rule out the possibility that you don't know why it was there for other people.
No, why it was there is a question for whoever put it there, and only them. You might ask other people why they didn't remove it, but that's a different question from why it was there.
Whoever put it there isn't answering any questions, and neither are the people who made it load-bearing later, if they're not alive anymore. No, the question is for you. If you owned this property, would you want to know about the work done by the people who came later and made it load-bearing, or would you rather remain unaware as you go about your renovations?
All models are false, some are useful. Knowing the whole history of everything that anyone has ever done in your property would be ideal, but impossible. Knowing why things were originally put in is a useful heuristic that strikes a good balance between simplicity and false positives/negatives.
