We can't live by bread alone or cornbread and buttermilk. It all has to be taken in and digested. They were to plan ahead only for the Lord's day. God would have them take no thought for tomorrow and constantly depend on Him for their daily bread. Daily we need God's spiritual food.
Life is more than food or clothing.
He tells us to look at the birds and the lilies of the field. They don't sow or reap or gather into barns, yet God takes care of them. Don't think on physical things, but seek first the kingdom of God and His righteousness, and all these shall be given unto you. So don't be anxious about tomorrow. Tomorrow will take care of itself tomorrow.