D&C 124

As the Lord instructs the saints to build a temple in Nauvoo, He explains - 

"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 124:28)

Therefore - 

"I command you, all ye my saints, to build a house unto me..."  (Doctrine and Covenants 124:31)

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

"Therefore, verily I say unto you, that 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, which my people are always commanded to build unto my holy name.

40 And 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, and the place whereon it shall be built."  (Doctrine and Covenants 124:39–42)

Without such - 

"...the whole earth would be utterly wasted at his coming."  (Doctrine and Covenants 2:3)