They're part of a the market making ecosystem. That market making allows everyone to buy/sell stocks easily at the true price. HFTs make that more efficient b

They're part of a the market making ecosystem. That market making allows everyone to buy/sell stocks easily at the true price. HFTs make that more efficient by reducing the bid/ask spread. We had market makers before HFTs so it's not an earth shattering innovation. But it is an incremental improvement.