Problem: This Greek word deo (“bound”) is later used for the binding of Satan during the millennium (Rev. 20:2). In the same way, these angels are said to be “bound” to the river Euphrates. Why?
Solution: In the OT, the river Euphrates was along the boundary of the Garden of Eden. When God made his covenant with Abraham, he gave him the land from the Nile to the Euphrates (Gen. 15:18). Osborne writes, “In 1 Enoch 56.5–57.3, the “angels of punishment” (56.1; cf. 53.3; 62.11) turn to the east to the Parthians and Medes and get them to attack Israel. This is very similar, for these angels are ‘bound at the great river Euphrates.’”
This language might be here to explain that God is binding the angels from doing more than they are permitted to do in his timing. Later, the Euphrates dries up so that the kings of the east could conquer the people there.
 Osborne, Grant. Revelation. Baker Exegetical Commentary on the New Testament. Grand Rapids, MI: Baker Academic. 2002. 379.
“And the sixth angel sounded, and I heard a voice from the four horns of the golden altar which is before God, (Rev 9:13)”