The giant represented a real and present danger to the people of God.  The giant despised the people of God and hurled curses upon them collectively.  The giant taunted the army of God with incredible insults and disarming lies.  No man in King Saul's army dared to challenge the giant in one on one combat.  However, God had a man who did challenge the giant, and faced him upon the battlefield.  Saul wanted the man of God to wear his armour, but Saul's armour represented the flesh, and David could not wear it.