Then David ran, and stood over the Philistine, and took his sword, and drew it out of the sheath of it, and killed him, and cut off his head therewith. When the Philistines saw that their champion was dead, they fled.
Another possibility is that Saul’s words could have been an idiom for, "What is his background?" Perhaps the writer included this reference to David’s family in the text because David’s trust and obedience resulted in his family enjoying special blessings from God through Saul. 1 Samuel 17:55-58 focus on the question of whose son David was. This event proved that David was a true son of God who had the reputation and interests of his Father and his Father’s people at heart (cf. John 8:29).