Jamieson, Fausset and Brown on 1 Samuel 9:15-16

15-16. Now the Lord had told Samuel in his ear a day before--The description of Saul, the time of his arrival, and the high office to which he was destined, had been secretly intimated to Samuel from heaven. The future king of Israel was to fight the battles of the Lord and protect His people. It would appear that they were at this time suffering great molestation from the Philistines, and that this was an additional reason of their urgent demands for the appointment of a king (see 1Sa 10:5; 13:3).