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 were 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 her?
Ramban and Seforno: He was ordering her to remain under the jurisdiction of her mistress Sarah.
A sign that her descendants would remain forever under the jurisdiction of Sarah's children.