Much is written in various biblical books about angels accompanying people in different and especially difficult situations. As for the Book of Daniel, it stands out especially among these books. This is not surprising: it was written in the second century BC, during the persecutions of the Syrian king Antiochus Epiphanes and the Maccabean wars that became the response to them.
The image of the martyr was very relevant then: the persecutions of the Synagogue organized by the Syrian authorities led to the appearance among believing Jews of martyrs and confessors. The picture of Babylon in the Book of Daniel looks somewhat artificial and stylized, which is not surprising: for the people of the age of the Antiochene persecutions and Maccabean wars, the Babylonian captivity was a distant past. But something else remained relevant: God's direct intervention in the situation when the fate of His people is at stake. The very image of God's messenger staying with the martyrs in the fiery furnace looks like a fairy tale, and the whole story of the three youths thrown into the fiery furnace reads like a legend.
But biblical authors often used not only their own revelations or historical facts to bear witness to God, but also legends, traditions, and myths. There is nothing surprising in this: history too was needed by them not for history's own sake. In history the authors of the biblical books sought and found what, even having happened in a definite place at a known time, turned out to be relevant for all times.
That is why even those books of the Bible that we habitually call historical can be called so only conditionally. They set forth far from all historical facts, even those that directly concern the characters mentioned in these books. On the other hand, to illustrate what had been revealed to them by God, the authors of biblical books could use episodes taken from legends or myths as freely as they used historical material.
There is no deception here: none of the biblical authors ever promised to write a detailed and absolutely reliable history of the Jewish people or of any other people. The books that we traditionally call historical are not accidentally called the Former Prophets in the Jewish tradition: even here there is no history in its pure form, but a historical illustration of what the later prophets, from Amos to Malachi, said about pre-exilic Jewish history.
Still less did the author of the Book of Daniel intend to write historical treatises. His angel becomes a symbol, a symbol of God's presence where His witnesses bear witness to God. Even, and especially, when they have to bear witness in a furnace blazing with fire.