Well I guess it’s easier to spend 10% of your wealth if you know you still have $4 billion left.
glib answer: More money than sense.
Edit: I realize you didn't have "why would you" in your question and answered something you didn't ask. Apologies. (I nontheless maintain my statements :) )
I can't even pretend to understand how people in that "wealth bracket" think. Imagine if you knew, or at least knew of, every person in the world with as much or more money than you (i.e. a few hundred). Imagine if you had enough money to ensure that, barring major wars or mistakes, all of your close friends and descendants could live comfortably without ever having to work another day. What would you do with the spare hundreds of millions of dollars? Build space ships? Build islands? Buy paintings? Buy paintings, only to set them on fire to demonstrate your power?
> (it's Davinci, which even for a pleb like me would be neat to have in your living room)
Imagine the maintenance costs. I'd keep the thing in a Swiss vault, and sell it to the highest bidder.