Gold has a lot of practical applications now (although is of course still treated as a ‘precious’ metal of value), but hundreds of years ago it was just a shiny metal. Why did it demand value, because of it’s rarity? Why not copper, because it was too easily found? What made it valuable ahead of other similar metals?
It was valuable because enough people accepted that it was valuable. It’s really that simple. Our money is backed by “the full faith and credit of the US government”. What is that really worth?
This begs the question “why did enough people start accepting that it was valuable?”
Rarity and shininess
When a few rich people say this is valuable the government has to protect their property because the government works for rich people. Once the government agrees then everyone else has to play along whether they think it’s valuable or not. Think crypto or fine art. You think bitcoin is stupid and worth nothing yet two rich people agree it’s worth 100000 then government enforces property rights. So as long as you fall under said governments jurisdiction that bitcoin is valued at 100000 whether you believe it or not.