From The Old Testament World by John Rogerson and Philip Davies:
To illustrate what Job is up against, God invokes his creative power. This, as we have seen in Proverbs, is an argument especially dear to Wisdom literature, for the maker of the universe is the source of all ethics too. But here the order in creation is definitely not the argument—rather the opposite! God does not present himself as a grand designer of a magnificent, orderly system. Instead he speaks of himself as one who created monstrous animals like the crocodile and the hippopotamus (Leviathan and Behemoth). Let a person understand these creatures, mightier than human beings! For if one cannot understand even these, how can one understand God? Job has been challenging a God of order and of justice. God responds as one whose ways do not make sense—at least to humans. One cannot ‘draw out’ a crocodile, and one cannot ‘draw out’ God in debate, either. One can only fear these terrible beasts, and fear God, who conceived and made them.
So Job accepts, and the poem undermines any complacency that wisdom might induce, any security in the ultimate reasonableness of life, or of God. The poem affirms God as a free agent, answerable to no-one, nor to any principle such as justice. But in the opening narrative we are told that Job’s suffering does have a rational basis, and God’s behaviour does make sense. Job, of course, knows nothing of this, and God does not speak of them, even in the closing narrative. So the reader of the book knows more than Job does, and more than God admits to Job. For God has been challenged by the Satan to a test (1:8-12; 2:3-6), a wager, and he has accepted. Job’s sufferings will determine whether righteousness really exists. In the story the test is a test not of Job but of God. And Job, not God, is the free agent.