If the major purpose of all scripture is to bear testimony of Christ, (and I believe it is) then the Doctrine and Covenants makes a remarkable contribution to that testimony. With few exceptions, everything that the Doctrine and Covenants teaches us about the Savior is taught in the actual words of the Savior. Almost all scripture is in some way about Christ. But the Doctrine and Covenants is by Christ. The words are His words. The voice is His voice. Most of the information in the Doctrine and Covenants is given in the first person singular by the Lord Jesus Christ Himself.