It's ability to do things and have an impact on the world. You can afford to try out crazy projects that require enormous amounts of capital. Obviously no one needs this, but there are plenty of things to spend your money on that isn't buying 30 of the same thing.

To someone in the bottom 10%, I'm sure they think there's no appreciable difference between a $100k/y annual income and $1M annual income. How much rent and food can one person consume, after all?

As others in the thread are trying to explain, there is a huge stratification at those levels. A $1B net-worther has a very different ability to project his will onto the world than a $.1B net-worther, or a $10B net-worther. That matters to them.