Know that the difficulties which lead to confusion in the question what is the purpose of the Universe or of any of its parts, arise from two causes: first, man has an erroneous idea of himself, and believes that the whole world exists only for his sake; secondly, he is ignorant both about the nature of the sublunary world, and about the Creator's intention to give existence to all beings whose existence is possible, because existence is undoubtedly good.
In the realm of Nature there is nothing purposeless, trivial, or unnecessary.
At times the truth shines so brilliantly that we perceive it as clear as day. Our nature and habit then draw a veil over our perception, and we return to a darkness almost as dense as before. We are like those who, though beholding frequent flashes of lightning, still find themselves in the thickest darkness of the night.
This must be our belief when we have a correct knowledge of our own self, and comprehend the true nature of everything; we must be content, and not trouble our mind with seeking a certain final cause for things that have none, or have no other final cause but their own existence, which depends on the Will of God, or, if you prefer, on the Divine Wisdom.
The philosophers likewise assume that in Nature there is nothing in vain, so that everything that is not the product of human industry serves a certain purpose, which may be known or unknown to us.
Know that for the human mind there are certain objects of perception which are within the scope of its nature and capacity; on the other hand, there are, amongst things which actually exist, certain objects which the mind can in no way and by no means grasp: the gates of perception are closed against it.