It is explained here in God’s Word:
Ephesians 4:11-14: And he gave the apostles, the prophets, the evangelists, the shepherds and teachers, to equip the saints for the work of ministry, for building up the body of Christ, until we all attain to the unity of the faith and of the knowledge of the Son of God, to mature manhood, to the measure of the stature of the fullness of Christ, so that we may no longer be children, tossed to and fro by the waves and carried about by every wind of doctrine, by human cunning, by craftiness in deceitful schemes.
It is to equip Christians for God’s work, which builds the body of Christ (God’s spiritual Kingdom that is not physical). It is done until perfection arrives (either we pass this temporary world, or our Lord returns to judge the world). In the meanwhile we (Christians) should grow in the faith so that the world will not be able to topple us down with its ever increasing assaults.
I like being reminded of the purpose. It is a good reminder, always.