Diversity of statuses and offices in the Church pertains to three things. The very first, to completeness of the Church itself. For just as in the order of natural things, completeness, which in God is found simply and uniformly, in the universe of creatures is not able to be found except differently and plurally, so also the fullness of grace, which in Christ as in the head is united, to the members overflows in many ways, so that the body of the Church may be complete. And this is what the Apostle says, Eph. 4: He gave some as apostles, some as prophets, others as evangelists, and others as shepherds and teachers, to the fulfillment of the saints.
And secondly it pertains to the the necessity of the actions which are necessary in the Church. For it is fitting for diverse actions to be deputed to diverse men, in order that all may be accomplished readily and without confusion. And this is what the Apostle says, Rom. 12: As in one body we have many members, and all members do not have one act, so we many are one in Christ.
And thirdly, it pertains to the the dignity and beauty of the Church, which consists in a certain order. Wherefore it is said, 1 Kgs 10, that the Queen of Sheba on seeing the wisdom of Solomon, and the rooms of his servants, and the orders of his ministers, had no more spirit. Wherefore also the apostle says, 2 Tim. 2, that in a great house there are not only vessels of gold and silver, but also of wood and clay.
[Aquinas, Summa Theologiae 2-2.183.2 corpus, my rough translation.]
