Today is the festival of St. Michael and All Angels. Here is the Treasury's explanation of this day:
The name of the archangel St. Michael means "Who is like God?" Michael is mentioned in the book of Daniel (12:1), as well as in Jude (v. 9) and Revelation (12:7). Daniel portrays Michael as the angelic helper of Israel who leads the battle against the forces of evil. In Revelation, Michael and his angels fight against and defeat Satan and the evil angels, driving them from heaven. Their victory is made possible by Christ's own victory over Satan in His death and resurrection, a victory announced by the voice in heaven: "Now the salvation and the power and the kingdom of our God and the authority of His Christ have come" (Revelation 12:10). Michael is often associated with Gabriel and Raphael, the other chief angels or archangels who surround the throne of God. Tradition names Michael as the patron and protector of the Church, especially as the protector of Christians at the hour of death.
Johann Gerhard gives four reasons why angels readily serve the believers though they are much nobler and higher than we:
1) Because they are confirmed int he good and therefore gladly and fully obey God's will. God's will and order is that they serve us (Heb. 1:14). The army of the heavens--sun, moon, and stars--maintain their order given them by God for man's sake. All the more will the heavenly army of the holy angels maintain its order.
2) Because our nature is raised in Christ above all angels and archangels (Eph. 1:20, 21; Heb. 1:4). Therefore the angels do not refuse to serve us men, in honor of the human nature assumed by Christ. As an entire race is brought to honor by a marriage, so the marriage of the Son of God with humanity has restored the human race to honor (Matt. 22:2). What wonder, then, that the angels serve us, since the Son of God, the Lord of the angels, came to earth that He might serve us?
3) Because love is pure and perfect in them, the angels joyfully serve us, as does the Lord, who Himself is Love (1 John 4:8), in whose image the angels were created, and who declared: "I will rejoice in doing them good" (Jer. 32:41).
4) Finally, because we shall someday be with them in heaven and join their choir in praising God, the angels are happy to serve us here on earth
(Treasury, p. 767.)