They reflect on the nature of promise (especially in reference to God’s call to Abram in Genesis 12).
The nature of God’s promise to Abram is that it goes against all human expectations. To the human eye, the land is full and Sarai is barren. In order that the land should be inherited by his offspring (seed) – not just the one who inherits – the tension of the story lies around these impossible descendants, the impossible child.
Impossible to human eyes, but not in God’s. There is no mention of curse or disfavour in relation to Sarai’s barrenness. It is merely the situation that she – and her family – are in. It’s a non-starter, but God is never troubled by such things. He can mend her broken womb.
Of course, Abram may have assumed that this offspring would come through another woman (and certainly this is what they later attempt, to make their own muddled version of the promise) – but God has always had Sarai – Sarah – in mind.
Sarah’s child was an impossible child, especially as the years went by and both she and Abraham were old and past childbearing. Isaac was not the only impossible child, of course, for there was one to come who defied every single possibility.
God loves doing impossible things – things that we cannot conjure for ourselves, things that flash pure divine brilliance across the skies of our lives.