Please teach me what purpose HFT serves to our economy.
This question has been beaten to death every time a HFT related article is posted. But people still ask, so I will try to answer. High Frequency Traders (HFTs) are not investors, they are market makers. They find a willing buyer and a willing seller, arrange the transaction, and execute the trade. They make a profit on the spread between the buy price and the sell price. The problem is that once they locate the buyer and seller, they need to buy the stock from the seller first, then turn around and sell it to the buyer, but the buyer may have cancelled they transaction, or they may have already bought the stock from someone else, in which case the HFT is stuck with the stock and may have to sell it to someone else at a loss. If transactions are granulated to one second intervals, instead of say, millisecond intervals, then the risk of this happening is a thousand times higher , and the HFTs will insist on higher spreads, resulting in lower liquidity and higher transaction costs for both buyer and seller.
Since the introduction of high frequency trading, transaction costs have fallen considerably, saving plenty of people a lot of money. The only losers are the old market makers that used to have lucrative sweetheart deals with the exchanges. Many of those old market makers are now bankrupt. Good riddance.