The Lord chooses who will be the President of the Church, who holds keys of the mysteries, and only he receives revelations for the Church, until he dies and the Lord appoints a new Church President in his stead.
Doctrine and Covenants 28:2, 6–7
2 But, behold, verily, verily, I say unto thee, no one shall be appointed to receive commandments and revelations in this church excepting my servant Joseph Smith, Jun. [or any other President of the Church], for he receiveth them even as Moses. . . .
6 And thou shalt not command him who is at thy head, and at the head of the church;
7 For I have given him the keys of the mysteries, and the revelations which are sealed, until I shall appoint unto them another in his stead.
Elder Bruce R. McConkie said:
“I can suppose that when the Prophet Joseph Smith was taken from this life the Saints felt themselves in the depths of despair. To think that a leader of such spiritual magnitude had been taken from them! Our revelation says, ‘Joseph Smith, the Prophet and Seer of the Lord, has done more, save Jesus only, for the salvation of men in this world, than any other man that ever lived in it’ (D&C 135:3). We do not have language or capacity or ability to extol the greatness and the glory of the ministry and mission of the Prophet Joseph Smith. And yet when he was taken the Lord had Brigham Young. Brigham Young stepped forth and wore the mantle of leadership. With all respect and admiration and every accolade of praise resting upon the Prophet Joseph, still Brigham Young came forward and did things that then had to be done in a better way than the Prophet Joseph himself could have done them.”
(Excerpt from “Succession in the Presidency,” Brigham Young University devotional address, Jan. 8, 1974.)