D&C 109

After having obeyed the revelations and instructions from the Lord, even in much tribulation, to build Him a house where He may manifest Himself, the Prophet Joseph Smith, in a revealed dedicatory prayer, prays -

"And now, Holy Father, we ask thee to assist us, thy people, with thy grace, in calling our solemn assembly, that it may be done to thine honor and to thy divine acceptance;
11 And in a manner that we may be found worthy, in thy sight, to secure a fulfilment of the promises which thou hast made unto us, thy people, in the revelations given unto us..."  (Doctrine and Covenants | Section 109:10 - 11)

In previous revelations, the Lord had given great promise if they would be obedient and build this Holy House.  And thus the Prophet pleads -

"That thy glory may rest down upon thy people, and upon this thy house, which we now dedicate to thee, that it may be sanctified and consecrated to be holy, and that thy holy presence may be continually in this house;
13 And that all people who shall enter upon the threshold of the Lord's house may feel thy power, and feel constrained to acknowledge that thou hast sanctified it, and that it is thy house, a place of thy holiness.
14 And do thou grant, Holy Father, that all those who shall worship in this house may be taught words of wisdom out of the best books, and that they may seek learning even by study, and also by faith, as thou hast said;
15 And that they may grow up in thee, and receive a fulness of the Holy Ghost, and be organized according to thy laws, and be prepared to obtain every needful thing;
16 And that this house may be a house of prayer, a house of fasting, a house of faith, a house of glory and of God, even thy house;
17 That all the incomings of thy people, into this house, may be in the name of the Lord;
18 That all their outgoings from this house may be in the name of the Lord;
19 And that all their salutations may be in the name of the Lord, with holy hands, uplifted to the Most High..."  (Doctrine and Covenants | Section 109:12 - 19)

These had been the promises if the saints did build this house and remained faithful and worthy to enter therein -

"...that no unclean thing shall be permitted to come into thy house to pollute it..."  (Doctrine and Covenants | Section 109:20)

And if by chance one fails to remain clean or -

"...when thy people transgress, any of them, they may speedily repent and return unto thee, and find favor in thy sight, and be restored to the blessings which thou hast ordained to be poured out upon those who shall reverence thee in thy house."  (Doctrine and Covenants | Section 109:21)

And with all these promises, the Prophet again reminds us of another promise -

"...we ask thee, Holy Father, that thy servants may go forth from this house armed with thy power, and that thy name may be upon them, and thy glory be round about them, and thine angels have charge over them..."  (Doctrine and Covenants | Section 109:22)

This is why we build temples.