Let's set aside arguments of individuality vs. socialism and whether government should operate for the hugely debatable, ephemeral term, "the greater good". I think that invites a separate essay on its own.
From a purely utilitarian perspective, the rich often leverage their capital to achieve things that would be impossible or difficult to do without access to the capital they created.
Case in point, is Elon Musk. If he didn't take home most of the money he earned from his involvement in Paypal, the odds of Space X, Tesla, and Solar City ever happening are decreased dramatically. When in the hands of an extraordinarily talented individual like Musk, capital can work to create long-lasting ripple effects that benefit society as a whole.
If you had taxed Musk at 80%, and he was left with a paltry $30 million or so, it's likely that one or more of the above companies never happen. The opportunity cost is that less jobs get created, unique and innovative technology never gets formed, and the ripple effects (such as spurring Chevrolet and others to make electric vehicles) never happen.
While not always the case, it is often the case that private capital investments do improve society in meaningful ways.
The economy is a very fungible thing and the idea that it is approaching "zero sum" is an unfounded one in the extreme.