D&C 84

As the Lord revealed doctrine of the Priesthood, we first learn that that the priesthood -

"...is without beginning of days or end of years."  (Doctrine and Covenants | Section 84:17)

And that two of the purposes and functions of the Melchizedek priesthood is  -

"...this greater priesthood administereth the gospel and holdeth the key of the mysteries of the kingdom, even the key of the knowledge of God."  (Doctrine and Covenants | Section 84:19)

The first purpose and function of "Administereth the gospel" suggests to me that the priesthood administers the work of salvation, being as outlined in the Handbook of Instruction, the -

"...work of salvation includes member missionary work, convert retention, activation of less-active members, temple and family history work, and teaching the gospel." (Handbook of Instruction - Book 2, Section 5

I would add, caring for the poor and the needy, and performing the saving ordinances and blessings of the work of salvation for -

"The power and authority of the higher, or Melchizedek Priesthood, is to hold the keys of all the spiritual blessings of the church—"  (Doctrine and Covenants | Section 107:18)

The second purpose of "holdeth the key to the mysteries" suggests to me that the Melchizekek priesthood brings with is the necessary power and -

"...the privilege of receiving the mysteries of the kingdom of heaven, to have the heavens opened unto them..."  (Doctrine and Covenants | Section 107:19)

Even to the point that the priesthood is the key to all the knowledge of God or in my mind, the knowledge that God possesses.  It it common to suggest that knowledge is power.  But associated to God's knowledge is God's righteousness and holiness...or in other words, godliness -

"Therefore, in the ordinances thereof, the power of godliness is manifest."  (Doctrine and Covenants | Section 84:20)

As the priesthood administers the saving ordinances of the gospel, as the saints follow in faith the doctrine of Christ, repenting of their sins and receive the saving ordinances of baptism and receiving the Gift of Holy Ghost, and steadfastly enduring to the end, that a great power of godliness and holiness become apparent or is manifest.  And -

"...without the ordinances thereof, and the authority of the priesthood, the power of godliness is not manifest unto men in the flesh..."  (Doctrine and Covenants | Section 84:21)

Without acquiring a nature of holiness and godliness -

"For without this no man can see the face of God, even the Father, and live."  (Doctrine and Covenants | Section 84:22)

Or in my mind, to live is eternal life.  Joseph Fielding Smith wrote -

“It is the Holy Priesthood that unlocks the door to heaven and reveals to man the mysteries of the Kingdom of God. It is this Divine Authority which makes known the knowledge of God! Is there any wonder that the world today is groping in gross darkness concerning God and the things of his kingdom? We should also remember that these great truths are not made known even to members of the Church unless they place their lives in harmony with the law on which these blessings are predicated. (D. & C. 130:20–21.)” (Church History and Modern Revelation, Vol. 1, page 338.)

What a great blessing the restored priesthood is in our life.