Joseph, in a poem to W. W. Phelps, explains that authority is needed to seal up people to eternal life.
Joseph Smith, Poem, to William W. Phelps, in Times and Seasons 4, no. 6 (February 1, 1843): 84, The Joseph Smith Papers website, accessed September 29, 2021
43. For these overcome, by their faith and their works,
Being tried in their life-time, as purified gold,
And seal’d by the spirit of promise, to life,
By men called of God, as was Aaron of old.
Andrew F. Ehad comments: "Having been called to serve in the ministry by revelation of God through the prophet Moses (Exodus 28:1), Aaron became a type of how these blessings would be controlled. The prophet might not always administer these blessings personally. But each nominee could only be cleared for these ordinances by the prior, official approval of the prophet" (Ehat, Andrew F. "Joseph Smith's Introduction of Temple Ordinances and the 1844 Mormon Succession Question." M. A. Thesis, Brigham Young University, 1982, p. 83).