Dressing David in his tunic and armour, it is likely that Saul wanted to help prepare David for the fight. He expected David to engage in close range hand to hand combat and so he gave him the best equipment that he had.1 Alternatively, it is possible that Saul's tunic and armour were meant to identify/tie him to David so that he could share in a potential victory and take credit for it.2 When David refused, he denied Saul this opportunity.3
38 Then Saul clothed David with his armor. He put a helmet of bronze on his head and clothed him with a coat of mail,