Matthew 3

"IN those days came John the Baptist, preaching in the wilderness of Judæa,
2 And saying, Repent ye: for the kingdom of heaven is at hand.
3 For this is he that was spoken of by the prophet Esaias, saying, The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight." (New Testament | Matthew 3:1 - 3)

At that time, John the Baptist not only held the the lessor or Aaronic Priesthood, but also held the keys of administration of that priesthood. In modern revelation that the Lord -

"...took Moses out of their midst, and the Holy Priesthood also;
26 And the lesser priesthood continued, which priesthood holdeth the key of the ministering of angels and the preparatory gospel;
27 Which gospel is the gospel of repentance and of baptism, and the remission of sins, and the law of carnal commandments, which the Lord in his wrath caused to continue with the house of Aaron among the children of Israel until John, whom God raised up, being filled with the Holy Ghost from his mother's womb.
28 For he was baptized while he was yet in his childhood, and was ordained by the angel of God at the time he was eight days old unto this power, to overthrow the kingdom of the Jews, and to make straight the way of the Lord before the face of his people, to prepare them for the coming of the Lord, in whose hand is given all power." (Doctrine and Covenants | Section 84:25 - 28)

Elder Bruce R. McConkie wrote -

"Few prophets rank with John the Baptist. Among other things, his ministry was foretold by Lehi (1 Ne. 10:7-10), Nephi (1 Ne. 11:27; 2 Ne. 31:4-18), and Isaiah (Isa. 40:3); Gabriel came down from the courts of glory to announce John's coming birth (Luke 1:5-44); he was the last legal administrator, holding keys and authority under the Mosaic dispensation (D. C. 84:26-28); his mission was to prepare the way before, baptize, and acclaim the divine Sonship of Christ (John 1); and in modern times, on the 15th of May, 1829, he returned to earth as a resurrected being to confer the Aaronic Priesthood upon Joseph Smith and Oliver Cowdery." [JS-H 1:66-75; DC 13.] (MD, p. 393.)

And -

"This miraculously-born son of Zacharias was he last legal administrator of the old dispensation, the first of the new; he was the last of the old prophets, the first of the new. With him ended the old law, and with him began the new era of promise. He is the one man who stood, literally, at the crossroads of history; with him the past died and the future was born. He was the herald of the Messianic age, the messenger, forerunner, and Elias who began the great restoration in the meridian of time and on whose secure foundation the Son of Man himself built the eternal gospel structure. His ministry ended the preparatory gospel; Messiah's commenced again the era of gospel fulness."(DNTC 1:113.)

I sense John the Baptist fulfilled the very purpose of the "preparatory" preisthood in that he prepared the people for the Savior and the greater blessings of the Melchizedek priesthood for He did -

"...indeed baptize you with water unto repentance..." (New Testament | Matthew 3:11)

But also testified that -

"...he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptize you with the Holy Ghost, and with fire..." (New Testament | Matthew 3:11)

Of which he also testified -

"...he saw the Spirit of God descending like a dove, and lighting upon him:
17 And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased." (New Testament | Matthew 3:16 - 17)

It seems from that day on, John the Baptist understood he had fulfilled his role and stepped out of the way.