Much has been 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 B.C., 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 against the Synagogue organized by the Syrian authorities led to the appearance of martyrs and confessors among believing Jews. The picture of Babylon in the Book of Daniel looks somewhat artificial and stylized, which is not surprising: the Babylonian exile was a distant past for people of the age of the Antiochene persecutions and the Maccabean wars. But something else remained relevant: God's direct intervention in the situation when the destiny of His people is involved. The very image of God's messenger abiding with the martyrs in the furnace burning with fire 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, for witness to God, not only their own revelations or historical facts, but also legends, traditions, and myths. There is nothing surprising here: history itself was not needed by them for history's own sake. In history the authors of the biblical books sought and found what, even when it happened in a definite place at a known time, proved relevant for all times.
That is why even those books of the Bible which we habitually call historical can be called so only conditionally. Far from all historical facts are set forth there, even among those that directly concern the characters mentioned in those books. On the other hand, to illustrate what had been revealed to them by God, the authors of the biblical books could use episodes taken from legends or myths just as freely as they used historical material.
There is no deception here, for 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 we traditionally call historical are not accidentally called the Former Prophets in Jewish tradition: even here there is no history in pure form, but a historical illustration of what the later prophets, beginning with Amos and ending with 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 all the more, when they have to bear witness in a furnace blazing with fire.