King Saul asked Jesse to send David to him because he wanted David to serve him. After David had been anointed as the future king by the prophet Samuel, Saul was troubled by an evil spirit, and his servants suggested bringing in a skilled musician to help soothe him. David, who was known for his skill with the lyre, was brought to Saul to play music for him and to serve in the king's court.

1 Samuel 16:21 (RSV):

“And David came to Saul and entered his service. And Saul loved him greatly, and he became his armor-bearer.”