Iyov had rebutted all of his friends' answers on both questions of tzaddik vra lo and rasha vetov lo. Eliphaz responds with a new shitah that Hashem, in His wisdom, purposely does not reward the tzaddik, or punish the rasha immediately in this world. This is to give mankind the option of serving Hashem completely for His sake, and not only out of hope of reward or fear of punishment. When a person serves Hashem in hope of reward or fear of punishment, he is not serving Hashem, but is rather serving himself. This is not the kind of avodah which Hashem desires.
In addition, if reward and punishment were immediate, man would lack bechira and nisayon. Hashem's entire purpose in creating mankind was giving man the option to choose good over evil out of kavod for Hashem and His mitzvos. This is precisely why Hashem's hashgacha is covered, and hidden from the eyes of mankind. The concealment of hashgacha is what causes the "sechel" to deny Hashem's presence, and claim that the world runs systematically, subject to the whims of nature. All those who deny Hashem's supervision of man bring proof from the injustice of the affliction of tzaddikim and the serenity of reshaim. However, this discrepancy is simply a test to discern between a true servant of Hashem and one that serves Him for superficial reasons. A tzaddik who serves Hashem Lishma will live by his faith, and will not falter from his place. Eventually, he'll receive his reward in the world of everlasting spiritual reward because he was tested and found shalem. Likewise, the reshaim will receive their punishment in the next world There, everything hidden will become apparent, and Hashem will be revealed as a Kal Emunah without injustice.