Jamieson, Fausset and Brown on Ecclesiastes 3:18

18. estate--The estate of fallen man is so ordered (these wrongs are permitted), that God might "manifest," that is, thereby prove them, and that they might themselves see their mortal frailty, like that of the beasts.

sons of men--rather, "sons of Adam," a phrase used for "fallen men." The toleration of injustice until the judgment is designed to "manifest" men's characters in their fallen state, to see whether the oppressed will bear themselves aright amidst their wrongs, knowing that the time is short, and there is a coming judgment. The oppressed share in death, but the comparison to "beasts" applies especially to the ungodly oppressors (Psa 49:12, 20). They too need to be "manifested" ("proved"), whether, considering that they must soon die as the "beasts," and fearing the judgment to come, they will repent (Da 4:27).