Malbim: He comes to dispute what Iyov said (Perek 9) that individual conduct cannot come from Hashem, for there is injustice - Tzadik v'Ra Lo. Rather, He handed over conduct to the Ma'arachah; it does not distinguish between Tzadik and Rasha. Bildad said, this itself is unjust, to let the Ma'arachah consume a Tzadik without reason! Iyov answered, this is not evil considering the entire existence. Even if the entire earth would be lost, and all flesh would die, via the general nature, this is not unjust. The earth and all on it is like a grain of sand compared to the entire existence. Elihu comes to refute this.