The Sanctuary
When G-d told the Israelites that they should build a sanctuary for Him, the ultimate purpose that it served was that through its physical materials, G-dliness could dwell in the mundane.
This fulfills G-d’s ultimate intent for the creation of the world, in that it should become a dwelling place for His Presence.
Text 12
It is a well-known Rabbinic statement that the purpose of the creation of this world is that the Holy One, blessed be He, desired to have an abode in the lower worlds. But surely with Him the distinction of "upper" and "lower" has no validity, for He pervades all worlds equally...clearly, the purpose of the Hishtalshelut of the worlds and their descent, degree by degree, is not for the sake of the higher worlds, because for them this is a descent from the light of His blessed Countenance. But the ultimate purpose [of creation] is this lowest world, for such was His blessed will that He shall have satisfaction when the sitra achra is subdued and the darkness is turned to light, so that the Divine light of the blessed En Sof shall shine forth in the place of the darkness and sitra achra throughout this world, all the more strongly and intensely, with the excellence of light emerging from darkness.
Tanya, Likutei Amarim, Ch. 36
G-d’s intent for the world is to create a dwelling place for Himself in the lowest of places. For this to be accomplished, G-dliness must permeate every area—even those that were originally used for negativity.
This is expressed in the use of the mirrors for the Mishkan. Though created for the purpose of temptation (albeit for a good and holy intention of increasing Jewish progeny), G-d wanted holiness to specifically permeate the lowest levels. It is for this reason that G-d desired that they be included in the building of the Mishkan so that there too G-dliness should be expressed.
