A question about Book of Job?

I just read the Book of Job in cl a few days ago, but I do not quite understand what is the purpose of it. Does the book try to show that God is omnipotent and incomprehensible? Why is Job rewarded at the end after his repentance to God? Does that mean God is still the righteous? The Book of Job is included in the category "A critique of reason" in my cl. So does this book really criticize reason?