Why did God promise Abraham and Sarah a son?
God promised Abraham and Sarah a son because through that child would come the nation of Israel, fulfilling His covenant promises. The promised line was to be called in Isaac, not Ishmael, so the son was essential to those covenant promises.
Les explains that God had promised Abraham and Sarah a son for fifty years. The reason given is that out of that son would come the nation of Israel in fulfillment of the covenant promises. After about fifty years, Abraham became impatient and returned to pagan customs by using a surrogate mother. Sarah thought of it first and suggested they have a son by their slave girl Hagar, and Abraham quickly agreed. From that fleshly idea came Ishmael, Abraham’s natural son. But Paul makes clear that not all Abraham’s seed are children; rather, “in Isaac shall thy seed be called.” That distinction matters because the Arab world claims to be the true children of Abraham, and the Koran is understood to place the covenant promises with Ishmael instead of Isaac, which is why they feel they are the rightful owners of the Holy Land and why the Middle East conflict continues.