Final judgment belonged to the sphere of God's sovereign rule. Jewish apocalyptic expectation could include agents participating in judgment, but Matthew's description gives the Son of Man an extraordinary centrality.
The imagery of shepherding also evokes Israel's scriptural picture of God as shepherd who judges between members of his flock.
Daniel 7 joins the Son of Man with heavenly enthronement and universal dominion. Ezekiel 34 portrays the LORD as the shepherd who searches for his sheep and judges between sheep within the flock.
Matthew combines these themes as the glorious Son of Man separates the nations as a shepherd separates sheep from goats.
Jesus describes the Son of Man arriving in glory with all the angels and sitting on his throne. Every nation is gathered before him, and he personally separates the people into two groups.
The King then pronounces the final verdict. One group inherits the kingdom and enters eternal life; the other departs into eternal punishment. The criterion is expressed in terms of their treatment of those whom the King identifies with himself.
The passage goes beyond Jesus merely predicting that God will judge the world. Jesus places the Son of Man on the judgment throne and makes him the one who renders the final verdict over all humanity.
Universal judgment, command of angels, glorious enthronement, and determination of eternal destiny together form one of Matthew's strongest portraits of Jesus exercising divine authority.