It's an amazing feat to create islands when and where we want them and to build taller and taller buildings. We're getting better at it, too.
It is fairly pointless, on a global scale. To the developers, it's their retirement and then some. To the people who buy these places, it's keeping up with the Joneses and something to discuss at social gatherings. I'd say that the money will do some good. If you think about the amount of worker migrants who have gone to the UAE and are involved in the booming construction trade, then money is being moved around that part of the world quite well. They probably send money home to relatives, wives, family and so on, and the money is spent in their community.
The kind of people who have shitrakes of money to spend usually feel guilt-tripped or socially responsible to give money to charity, often to be seen giving money to charity, even if it's only to make themselves feel good.
Without the real estate, the islands, the glitz and the glam, I doubt any of these incredibly wealthy people would hand over any money. Without the construction, a lot of migrant workers would have less or no work, and I doubt money would spread as well to poor families throughout Africa the Middle East and Asia. Even if it isn't intentional, charitable effects are probably being seen more than if the construction didn't happen.