Not much it seems. No benefit to society at large.
Read the article about What Good is Wall Street here!
A quote:
A recent study by the research firm Ibbotson Associates shows that during the past decade investors in hedge funds, over all, would have done just as well putting their money straight into the S&P 500. “The amount of rent capture has been huge,” Woolley said. “Investment banking, prime broking, mergers and acquisitions, hedge funds, private equity, commodity investment—the whole scale of activity is far too large.” I asked Woolley how big he thought the financial sector should be. “About a half or a third of its current size,” he replied.