The Letter of the Law –vs- The Spirit of the Law

Why should we seek spiritual understanding of God's word? Isaiah said, No weapon that is formed against thee shall prosper (Isaiah 54:17). What does this really means? What does the Bible really mean when it says, honor your mother and your father and your days shall be long? (Exodus 20:12), Honor thy father and thy mother: that thy days may be long upon the land which the Lord thy God giveth thee. (Deuteronomy 5:16), Honor thy father and thy mother, as the Lord thy God hath commanded thee; that thy days may be prolonged, and that it may go well with thee, in the land which the Lord thy God giveth thee.