Angels reveal prophetic truth from God

 This comes out i Daniel 10 as well as in other places such as Ezekiel, Zechariah and Revelation.

Angels prevent harm coming to God's servants

In Daniel 6:22 Daniel says to the king "My God sent his angel, and he shut the mouths of the lions. They have not hurt me, because I was found innocent in his sight. Nor have I ever done any wrong before you, Your Majesty." Whether Daniel saw anything or not he certainly believed, and this is now in Scripture, that it was angels who keep the mouths of the lions shut so that they did not harm God's servant. No doubt angels often intervene so that God's servants are kept from harm.

Wesley comments on Daniel 6:22 "See the power of God over the fiercest creatures! See the care God takes of his faithful servants, especially when they are called to suffer for him! See how ready the angels are to minister to the heirs of salvation!"

Joseph Exell says "The text in Daniel suggests the whole doctrine of angel-ministration. That imperilled life guarded by a sentinel from heaven is no solitary spectacle. It has many parallels. There had been the ministration of angels before, as there has been a thousand times since."

Angels often startle people and make them afraid when they appear

Daniel 10:4-8  On the twenty-fourth day of the first month, as I was standing on the bank of the great river, the Tigris, I looked up and there before me was a man dressed in linen, with a belt of fine gold from Uphaz around his waist. His body was like topaz, his face like lightning, his eyes like flaming torches, his arms and legs like the gleam of burnished bronze, and his voice like the sound of a multitude. I, Daniel, was the only one who saw the vision; those who were with me did not see it, but such terror overwhelmed them that they fled and hid themselves. So I was left alone, gazing at this great vision; I had no strength left, my face turned deathly pale and I was helpless.
Luke 1:12 When Zechariah saw him, he was startled and was gripped with fear. 

Luke 2:9 An angel of the Lord appeared to them (the shepherds), and the glory of the Lord shone around them, and they were terrified.

Matthew 29:2-4 There was a violent earthquake, for an angel of the Lord came down from heaven and, going to the tomb, rolled back the stone and sat on it. His appearance was like lightning, and his clothes were white as snow. The guards were so afraid of him that they shook and became like dead men.

Gill notes how in the case of Zechariah he was troubled, and fear fell upon him; for such appearances of angels were not now so common as formerly: and when they were more usual, generally had such effects on the minds, even of good men and in the case of the shepherds - at the sight of such a personage, and at such unusual light and glory about them: they ... not used to such appearances ... were awed with the majesty of God, of which these were symbols, and were conscious to themselves of their own sinfulness and frailty.

Angels are capable of swift flight

Daniel 9:21 ... while I was still in prayer, Gabriel, the man I had seen in the earlier vision, came to me in swift flight about the time of the evening sacrifice.

Swift flight is no doubt a metaphor of some sort, a theriomorph of some description. What it identifies is the ability of Gabriel and other angels, no doubt, to travel speedily. Without the aid of machines or other helps of that sort they are able to cover huge distances in a short time.
This would seem to fit in with
Revelation 14:6 Then I saw another angel flying in midair, and he had the eternal gospel to proclaim to those who live on the earth - to every nation, tribe, language and people. 

Also Psalm 104:4 (quoted in Hebrews 1:14) He makes winds his messengers, flames of fire his servants.