With this action the angel demonstrates that the Lord God answers the prayers of the saints. Those under the altar (Revelation 6:9–10) had called out to ask God when he would avenge their blood. Those prayers are sweet‑smelling to God (recall the incense) and so pleasing to him—and so he answers. That is the purpose of the angel throwing fire on the earth.
5 Then the angel took the censer and filled it with fire from the altar and threw it on the earth, and there were peals of thunder, rumblings, flashes of lightning, and an earthquake.