This brief rhetorical questions and the short statement that follows it form the core message that impacts Job in the "answer" that God gives him. Over and over in God's speech to Job, The Lord has emphasized that He is sovereign over the universe. He controls everything, even the wildest of creatures and the strongest of storms. Job cannot take care of even a wild donkey in the desert. Job can't even send a breeze to blow where he wills it. God is God. Job is not.
But in this question God instructs Job that no one has ever given anything to God that He didn't already have. God owes no one. Ever. God already owns the universe. It is impossible for God to need anything from anybody. Even when we "give" Him our worship, we are giving Him what He deserves, and indeed already has. The universe is His estate. We are part of what He already has. And nobody is God's creditor. It is impossible.
Implicit in this truth is that God did not "owe" an answer to Job for his circumstances. God could do with Job as He wished. And to think otherwise is a form of pride. Ownership is part of Lordship.
However, the reality is that mercy brought Job this answer. God did not speak to Job out of obligation. He spoke out of His own will and love for Job. God delighted in Job from the very beginning of the book. And He came to Job in the tornadic storm out of His love for His servant.
You see, when this whole trial started with Job, it was God Who brought up Job's name first to Satan (Job 1:8). God was quick to "brag" on his exceptional servant. God was pleased with Job's life of obedience. And God was willing to let Satan test him to show that his delight in Job was justified. God wil sovereignly allow trials to those whom He loves. And He will show us His mercies when He does... not because we are owed an explanation, but because He delights in us at all times. This is reality both in and out of our difficulties.