This is the big Mormon temple in my town. It is enormous and it was built in the affluent area of town. When it was fully lit, it was a beacon from miles away and it might have become a town landmark.

But.

The affluent neighbors complained. Loudly.

They are not allowed to fully light the structure except for holidays now. They fought it, but they lost and it created new “light pollution” laws in our city.

Why they do it, is for that reason. To be a beacon to the faithful and to the lost. That’s part of their mission.

you think that is opulent?
Look at this

What would you do if a bunch of people gave you money and you didn’t have to pay taxes?

But @ragingloli I like those gothic cathedrals. That heavy, supportive architecture on the outside that allows an otherworldly, soaring expanse inside… that’s inspirational. That’s the feeling that the architect’s hands were guided. That is not what the modern Mormon temple does.

I wouldn’t go as far as to call them opulent, but they are certainly very nice. Ancient temples were also very costly (see the temple of Solomon in 2 Chronicles). This page provides some good answers. It’s basically the same reason why any religion builds beautiful houses of worship in dedication to their God. LDS temples are built to different sizes based on the need where they’re built. Some are larger, some are smaller.

Interstate 15 is what I call the “Mormon Corridor”. From where it breaks off of I-90 just North of Butte, down to SLC and beyond. Any town with enough population to be more than a ranch-exit, I say “look for the temple”. Sure enough, there is a shining light out there.

Because they tried building treehouses and none of them were very appropriate for holding church services?

I like to think they build expensive , granite temples because they believe that God deserves the very best.
I hope it isn’t because they want to show off.

LDS Apostle Bruce McConkie explained that a temple is “…a House of the Lord… where he and his Spirit may dwell … From the days of Adam to the present, whenever the Lord has had a people on the earth, temples and temple ordinances have been a crowning feature of their worship. ‘My people are always commanded to build temples: the Lord says, ‘for the glory, honor, and endowment’ of all the saints (D. & C. 124:39–40).

Sounds noble but were temples in use from Adam to Moses? Temples and temple ordinances from ‘Adam to the present” should be found in the Bible if LDS beliefs and practice conform “literally and completely” to it.

But until God revealed the design and use of the tabernacle to Moses in Ex. 25–40, neither tabernacles nor temples were even mentioned! Instructions to Moses for the tabernacle would have been unnecessary if temples were already in use!

The first time the word “temple” is used in the Bible is in I Sam. 1:9, where it refers to the portable tabernacle Israel received through Moses. The first permanent temple was built by King Solomon about 1,000 years before Christ (I Kgs. 6:17).

Does God dwell in temples built by men? Bruce McConkie said a temple is a “House of the Lord … where he and his Spirit may dwell:’ But Solomon questioned, “Who is able to build Him an house. Seeing the heaven and heaven of heavens cannot contain Him? Who am I, then, that I should build Him an house, save only to burn sacrifices before Him?” (II Chron. 2:6).

Why do Mormons build such opulent temples? First because they can. Second considering that God does not dwell in temples or buildings of any sort, Mormon temples are built for Mormons, not God. God is not impressed with opulent temples, cathedrals, or churches. He looks at those as subble to be burned.

God looks at what resides in a person’s heart not what can be built with their hands.