The angel says to Manoah’s wife, Behold, you are barren and have not borne children.
She would have known this, so why does he say this? Most likely, he wants to rouse her from her apparent apathy about it. He wants to get her attention. This explains why he begins with behold.
Another element here is that the Lord does not turn his attention away from his people, despite what they have done to him. God notices the situation of his people. His judgment and punishment for his covenant children do not last forever.
3 And the angel of the LORD appeared to the woman and said to her, “Behold, you are barren and have not borne children, but you shall conceive and bear a son.