A very challenging question that people ask about Revelation 16: What is the purpose of the seven bowl/plagues if they are after the close of probation and therefore no repentance can be expected? I think there are a number of considerations to keep in mind.
First of all, Revelation makes clear that God is not the author of death, pain and destruction (Rev. 7:1-3). Satan is the destroyer (9:11). Because freedom is essential in order for genuine love and trust to exist, it is crucial to the peace and security of the universe. But if people are free to love, they are also free to hate, rebel and harm. Respecting freedom means not only allowing creatures the freedom to choose, but allowing them to experience the consequences of their choices. A God who constantly intervenes to prevent negative consequences is not a God of freedom. So God allows Satan a certain freedom of action in the course of history and at the End, after securing the righteous, God allows Satan to more fully demonstrate what his kind of government would look like. One purpose of the seven last plagues is to convince the universe that Satan’s alternative to love and trust in God leads to total disaster. This will help convince free beings in the universe to never choose that option again.
Second, even Satan’s worst actions can be used by God to fulfill His purposes (17:17). The deceptions and plagues of the final crisis expose the truth about Satan and those who follow him (2 Thess 10-12). It is not God’s fault that the wicked are unredeemed, neither the grace of God (Rom 2:4) nor the plagues of the End (Rev 16:9, 11, 21) bring about any repentence. They are hardened in the course they have chosen. Thus, even the destruction of the wicked glorifies the character of God in the end (Rev. 15:3-4). They have made themselves unsafe to save and God sadly lets them go (Hos 11:7-8). Even after the millennium and a clear perspective on God’s character, nothing in their character has changed (Rev. 20:7-10). The plagues expose their settled unfitness for eternity and vindicate God’s judgment in each case.
So even though probation has closed, the seven last plagues serve a purpose in preparing the universe for a free, loving, safe and secure eternity.