"I believe it is wrong to view investors, including long-term investors like Buffett and short-term investors like hedge fund managers, as parasitical rent-seekers. Yes, they are motivated by self-interest, but by buying undervalued securities and selling overvalued ones, their actions bring the prices of capital assets closer to fundamental value. Those asset prices govern the allocation of investment resources, which in turn is paramount for the success of the economy. So, while making themselves rich, investors make the economy more productive for everyone--as if led by an invisible hand."
Gregory Mankiw, The Social Value of a Wall Street Career