Why does the Torah see fit to mention "'Mal'ach Hashem" each time the angel spoke to Hagar?
Rashi: Because each of the four statements was said by a different
Though it is not clear why this was necessary.
Ohr Yechezkel (Midos, p. 231): She merited seeing all these angels, for (bearing) disgrace and suffering makes one dear in Hashem's eyes.
What exactly was the angel telling Hagar?
Ramban and Seforno: He was ordering her to remain under the jurisdiction of her mistress Sarah.
This was a sign that her descendants would remain forever under the jurisdiction of Sarah's children (See also Or ha'Chayim).