פּירוש אויף בראשית 16:1

שפחה מצרית A HANDMAID, AN EGYPTIAN — She was a daughter of Pharaoh; when he saw the miracles which had been performed for Sarah’s take he said, “It is better for my daughter to be a handmaid in this man’s house than be mistress in another man’s house” (Genesis Rabbah 45:1).

ושרי אשת אברם לא ילדה, “meanwhile, Sarai, Avram’s wife had failed to give birth;” this verse is intended to inform us that the covenant just concluded between G-d and Avram preceded the time when Avram was blessed with biological offspring. We should not assume that here too the principle that the Torah need not report events in their chronological sequence could be applied. As a reminder of this, the Torah reports that up until now Sarai had not conceived or given birth. “ולה שפחה מצרית,” and she had an Egyptian maidservant; she was a usufruct maidservant which he was obligated to provide her sustenance and could not sell her.

"And Sarah 2 in the tradition, Sarah the wife of Abraham, and Sarah in the tribe of Issachar with Deborah. This teaches that she was important as a mother.