Objection 1. It would seem that the angels are not sent on works of ministry. For every mission is to some determinate place. But intellectual actions do not determine a place, for intellect abstracts from the "here" and "now." Since therefore the angelic actions are intellectual, it appears that the angels are not sent to perform their own actions.
Objection 2. Further, the empyrean heaven is the place that beseems the angelic dignity. Therefore if they are sent to us in ministry, it seems that something of their dignity would be lost; which is unseemly.
Objection 3. Further, external occupation hinders the contemplation of wisdom; hence it is said: "He that is less in action, shall receive wisdom" (Sirach 38:25). So if some angels are sent on external ministrations, they would seemingly be hindered from contemplation. But the whole of their beatitude consists in the contemplation of God. So if they were sent, their beatitude would be lessened; which is unfitting.
Objection 4. Further, to minister is the part of an inferior; hence it is written (Luke 22:27): "Which is the greater, he that sitteth at table, or he that serveth? is not he that sitteth at table?" But the angels are naturally greater than we are. Therefore they are not sent to administer to us.
On the contrary, It is written (Exodus 23:20): "Behold I will send My angels who shall go before thee."
I answer that, From what has been said above (Question 108, Article 6), it may be shown that some angels are sent in ministry by God. For, as we have already stated (43, 1), in treating of the mission of the Divine Persons, he is said to be sent who in any way proceeds from another so as to begin to be where he was not, or to be in another way, where he already was. Thus the Son, or the Holy Ghost is said to be sent as proceeding from the Father by origin; and begins to be in a new way, by grace or by the nature assumed, where He was before by the presence of His Godhead; for it belongs to God to be present everywhere, because, since He is the universal agent, His power reaches to all being, and hence He exists in all things (8, 1). An angel's power, however, as a particular agent, does not reach to the whole universe, but reaches to one thing in such a way as not to reach another; and so he is "here" in such a manner as not to be "there." But it is clear from what was above stated (110, 1), that the corporeal creature is governed by the angels. Hence, whenever an angel has to perform any work concerning a corporeal creature, the angel applies himself anew to that body by his power; and in that way begins to be there afresh. Now all this takes place by Divine command. Hence it follows that an angel is sent by God.
Yet the action performed by the angel who is sent, proceeds from God as from its first principle, at Whose nod and by Whose authority the angels work; and is reduced to God as to its last end. Now this is what is meant by a minister: for a minister is an intelligent instrument; while an instrument is moved by another, and its action is ordered to another. Hence angels' actions are called 'ministries'; and for this reason they are said to be sent in ministry.
Reply to Objection 1. An operation can be intellectual in two ways. In one way, as dwelling in the intellect itself, as contemplation; such an operation does not demand to occupy a place; indeed, as Augustine says (De Trin. iv, 20): "Even we ourselves as mentally tasting something eternal, are not in this world." In another sense an action is said to be intellectual because it is regulated and commanded by some intellect; in that sense the intellectual operations evidently have sometimes a determinate place.
Reply to Objection 2. The empyrean heaven belongs to the angelic dignity by way of congruity; forasmuch as it is congruous that the higher body should be attributed to that nature which occupies a rank above bodies. Yet an angel does not derive his dignity from the empyrean heaven; so when he is not actually in the empyrean heaven, nothing of his dignity is lost, as neither does a king lessen his dignity when not actually sitting on his regal throne, which suits his dignity.
Reply to Objection 3. In ourselves the purity of contemplation is obscured by exterior occupation; because we give ourselves to action through the sensitive faculties, the action of which when intense impedes the action of the intellectual powers. An angel, on the contrary, regulates his exterior actions by intellectual operation alone. Hence it follows that his external occupations in no respect impede his contemplation; because given two actions, one of which is the rule and the reason of the other, one does not hinder but helps the other. Wherefore Gregory says (Moral. ii) that "the angels do not go abroad in such a manner as to lose the delights of inward contemplation."
Reply to Objection 4. In their external actions the angels chiefly minister to God, and secondarily to us; not because we are superior to them, absolutely speaking, but because, since every man or angel by cleaving to God is made one spirit with God, he is thereby superior to every creature. Hence the Apostle says (Philippians 2:3): "Esteeming others better than themselves."
Source: Summa Theologica (New Advent)