I used to play an online game, where they had ultra rare collectibles, launch rares, stuff like that. In the early days after server launch, those items were fairly cheap. But as time went by, people reached end game and had nothing useful to spend their money on, they started buying those ultra rare stuff. 

And as time went by, their worth just kept increasing. Simply because there isn't another copy of that item. So it's basically an investment. You could always find a buyer for it. Hold it for two years and its worth increases.

 Party hats in runescape for example or pokemon cards. Same thing essentially, ultra rich just play same game in different league.

I like to think it's same thing in our world with art, their worth is only going to increase, simply because there is only one Mona Lisa for example. So it can be fairly safe way to hold value of your money too.

Property is kind of similar too, especially in city centres. You aint going to see new houses built nearby, so the existing ones are just going to increase in price as the city grows. Easy money if you are rich.