Jesus mention this because the Pagans worry about everything, food clothes, shelter, money,etc. The point Jesus is making is God is Omnipresent, all powerful; having unlimited power. He will supply all of our needs. As a Christian we are believers in God and what He will do for His children, as a Gentile they are non-believers, they do not believe until they see it right away. This has a positive effect in my life knowing that "my heavenly Father knows what I need" in my life to give Him the glory.