David was ordered to play the lyre for King Saul because Saul was suffering from an evil spirit that caused him distress, and it was believed that music could soothe or relieve the symptoms of madness or insanity. Saul’s servants suggested bringing in a skilful musician to play music to help calm him, and David was chosen because of his skill with the lyre and his divine favour (1 Samuel 16:14-23).

1 Samuel 16:23 (RSV):

“And whenever the spirit from God came upon Saul, David took the lyre and played it with his hand; so Saul was refreshed and was well, and the evil spirit departed from him.”