HFT is a way for market makers to compete among each other. When I want to buy stock in some company or other I need someone to sell it to me, and when I want to sell a stock I need someone to buy it. I might get lucky and someone like me might be looking to sell or buy at the same time, but that's unreliable. However, there are people who have some of pretty much every stock out there, and who are always ready to sell a share of ACME to me for $11.59 or buy it from me for $11.58. Because there's a granularity to the prices of stocks the market makers can't compete on price, so exchanges end up using speed to resolve who gets the trade. Hence HFT.