D&C 124

After Elijah had visited the Prophet Joseph Smith in the Kirtland Temple, he said -

"Therefore, the keys of this dispensation are committed into your hands..."  (Doctrine and Covenants | Section 110:16)

Now having those keys restored, the Lord instructs the Prophet to -

"...build a house to my name, for the Most High to dwell therein.
28 For there is not a place found on earth that he may come to and restore again that which was lost unto you, or which he hath taken away, even the fulness of the priesthood."  (Doctrine and Covenants | Section 124:27 - 28)

In my mind, "the fulness of the priesthood" would mean the charge and privilege of exercising all rights, power, authority, covenants and ordinances restored in this dispensation.  Much of this restored authority, especially the sealing power and authority restored by Elijah, was to be exercised only in sacred temples -

"For therein are the keys of the holy priesthood ordained, that you may receive honor and glory."  (Doctrine and Covenants | Section 124:34)

In speaking of the work for the dead including baptisms for the dead -

"For a baptismal font there is not upon the earth, that they, my saints, may be baptized for those who are dead—
30 For this ordinance belongeth to my house..."  (Doctrine and Covenants | Section 124:29 - 30)

As well as -

"...your anointings, and your washings, and your baptisms for the dead, and your solemn assemblies, and your memorials for your sacrifices by the sons of Levi, and for your oracles in your most holy places wherein you receive conversations, and your statutes and judgments, for the beginning of the revelations and foundation of Zion, and for the glory, honor, and endowment of all her municipals, are ordained by the ordinance of my holy house..."  (Doctrine and Covenants | Section 124:39)

Even adding -

"...verily I say unto you, let this house be built unto my name, that I may reveal mine ordinances therein unto my people;
41 For I deign to reveal unto my church things which have been kept hid from before the foundation of the world, things that pertain to the dispensation of the fulness of times.
42 And I will show unto my servant Joseph all things pertaining to this house, and the priesthood thereof..."  (Doctrine and Covenants | Section 124:40 - 42)

People wonder why "Mormons" build temples.  This is why Mormons build temples...to receive the "glory, honor, and endowment" of higher sacred ordinances, covenants, and blessings.