Why is a study of the Doctrine and Covenants so important?

The Doctrine and Covenants is unique among the standard works of The Church of Jesus Christ of Latter-day Saints. It was revealed in this dispensation and is intended primarily for people who will live on the earth during this dispensation.
The Old Testament was revealed to prophets in ancient times primarily to prepare the people of those days for the first coming of the Savior. Gospel principles in the Old Testament, of course, are beneficial and instructive for all people of all ages.
The New Testament was written as a witness and testimony of the divinity of Jesus Christ, and as such it is important for all people of all times. The latter part of the New Testament, however, consists of letters and other instructions concerned primarily with the branches and members of the Church in the meridian of time.
The Book of Mormon was written and compiled by prophets in ancient times, but this book is primarily intended for people of this dispensation as a second witness of the Bible and of the divinity of Jesus Christ.
The Pearl of Great Price contains some of the writings and teachings of ancient prophets as well as the history and testimony of Joseph Smith and doctrinal instructions by other prophets of this dispensation. More than any of the other canonized scriptures, the Doctrine and Covenants has largely been revealed to and written by the prophets of this dispensation for the instruction and edification of the people of these times.

Concerning the importance of the Doctrine and Covenants, President Joseph Fielding Smith has written:

...This Doctrine and Covenants contains the word of God to those who dwell here now... More precious than gold, the Prophet says we should treasure it more than the riches of the whole earth. I wonder if we do? If we value it, understand it, and know what it contains, we will value it more than wealth; it is worth more to us than the riches of the earth. (Joseph Fielding Smith, DS 3:198-99.)