Why do we fight and argue about free will vs predestination, young earth vs old earth, etc? Why does it matter how God rules the universe when all that matters is that God does rule the universe?

Oh, but there are a lot of things about how God rules the universe that are absolutely central to Christianity. Would you say that it doesn't matter, for example, whether or not God rules the universe in part through the Holy Spirit, or whether he ruled the universe by making his son incarnate and sending him to be crucified and resurrected? After all, you could easily believe that God rules the universe without holding any particular opinion on those things. But those questions are undeniably important to the Christian faith.

I think you're drawing a false dichotomy when you say that we don't need to know all the answers. I doubt any Christian thinks that we should have all the answers. But by definition, a Christian will believe that we have some of them. So what people are disagreeing about is not whether we should have answers, but what the answer is where they think we do know the answer. For example, the Catholic Church teaches that Mary was protected from the moment of her conception from the stain of original sin. To a non-Catholic, this may seem like the sort of thing that we have no reason to worry about. But if you're a Catholic, you'll think this claim was revealed by God to the Church. That would make it pretty important, no?