I do not believe we can expact an answer to the question of why anything happens, nor do I see how the answer to this question has any relevance for our understanding of things. A question of purpose should be properly phrased in the context of consciousness, which itself is an emergent feature of the evermore complex nature of the brain. By all accounts advanced brains are a relativeley small scale phenomenon of the cosmos. But even a grand design of purpose, would not, I propose, give us a satisfying answer to the question. Consider a univers that just exists, and one brought into the existence of a creator who just exists. Both seem equally meaningless to me.
source, youtube > AndromedasWake > Atheist
There are more than one meaning of the word "why". One is final cause, i e for what purpose. But there is also efficient cause, i e by what driving force, material cause, i e from what matter, and formal cause, i e of what not just shape but actually nature.
As a Christian I believe all of these, not just the question of purpose require God to be correctly answered. God being efficient and final cause of all things (i e His purpose with creation is Himself, to realise His capacity of sharing His Glory but also that anything that produces any effect whatsoever ultimately does so because God is producing all effectivity). God is not identical with material or directly with formal cause of created things as the Pantheists think, however.
Now, there is a very important difference in explanation between a universe that just is there and a universe created by a creator that just is there, at least if the universe that just is there is the one you describe, in which mind is a byproduct of brain matter.
No science even begins to explain how mind can be a byproduct of the material "information processing" roughly parallel to that done by computers. It is basically, and I have said it before, as if beads on an abacus were to gain, as an emergent feature of abacus counting, an understanding of mathematics. They do not, they only illustrate our understanding of mathematics. Same is true for computers. Same would be true if brains were only matter and mind no primary.
In other words: if mind is no primary, mind cannot exist, because mind cannot be an emergent feature.
That is exactly where a universe created by a "mind that just is" explains more than a mind emerging from "matter that just is".
Now, if we are to correctly refute not just atheism, but pantheism as well, we may add that if God were identical to the material cause of universe and to the forms inherent in material objects, there would not be any other mind than God's. And then the supreme mind would be in error each time anyone thought something erroneous, or the erroneous thought would not be a thought, both explanations being absurd.
Since there are other mind's than God's, the universe is not identic to God, but created by Him.
You said yourself in the continuation of those words, that a universe created by divine purpose is one where we can never fully understand the purposes of so many arbitrary choises God made.
However, if you recall that starry night, you can be sure that beauty was one of them, supposing my explanation to be correct. Supposing it not to be correct, I would like to see a refutation of this essay.
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Update 29-IX-2015 : the video I quoted and linked to is now private.