The Temple was extravagant: cedar timbers, bronze altars, purple curtains, lots of gold. It was built to honour God: the God above all gods.
2 Chronicles 2:5-6 This must be a magnificent Temple because our God is greater than all other gods. But who can really build him a worthy home? Not even the highest heavens can contain him! So who am I to consider building a Temple for him, except as a place to burn sacrifices to him?
Solomon was very rich. There is an argument that, at the time, there was no better thing to do with his (and his nation’s) wealth than build a suitably extravagant Temple. Even so, he was following the plans David had passed on:
1 Kings 5:5 So I am planning to build a Temple to honor the name of the Lord my God, just as he had instructed my father, David. For the Lord told him, ‘Your son, whom I will place on your throne, will build the Temple to honor my name.’
Somewhere in the seven years it took to build it, God reminded Solomon of what was really important, and it wasn’t a building:
1 Kings 6:11-13 Concerning this Temple you are building, if you keep all my decrees and regulations and obey all my commands, I will fulfill through you the promise I made to your father, David. I will live among the Israelites and will never abandon my people Israel.
If you’re having trouble imagining what the temple looked like, a Baptist Bible College site has a good overview of the physical temple and the various interpretations that people have made.