6. What is the point to adding billions to multi-billion dollar fortunes?
A few billionaires, like Elon Musk (founder of PayPal, Tesla Motors and SpaceX), are using their billions to do interesting things and perhaps things beneficial for society in the long term. Many are in it just for the sport of making more money. The money is just for keeping score, not for living on (that requires very little of their incomes), nor for improving the world.
I suppose the Koch Brothers and Sheldon Adelson would insist the hundreds of millions they spent (wasted) on Mitt Romney's campaign were meant to save the world from the evils of Obama-ism, not as an investment for which they would have demanded Romney provide them with massive profits, nor as an indulgence of their own egos. They would be lying to us and possibly to themselves by saying so.
Guys like Donald Trump, though, seem interested only in making the next deal. And the deals have to get bigger and bigger to continue to provide the "rush" they crave.