שמואל א, פרק ט״ז, פסוק י״ט

Accepting the advice of his court, Saul dispatches messengers to Jesse. For the king, reaching out to a citizen to recruit their son was a completely natural and routine action. He regularly sought out talented individuals to join his royal court and fight in his military campaigns [ביאור שטיינזלץ].

The message delivered to Jesse is a direct summons for his son. However, this request is only preliminary. At this stage, Saul simply wants to meet the young man and evaluate his skills. He does not yet know if he will make a good impression and secure a permanent position in his service [מלבי״ם].

To ensure Jesse knows exactly which of his children is being summoned, the king specifically identifies the son who is out with the flock, tending and caring for the sheep [מצודת דוד, ביאור שטיינזלץ].