I'm not talking about the bid-ask spread. A bid is just someone willing to buy something for a specific price. An ask is someone willing to sell something at a

I'm not talking about the bid-ask spread. A bid is just someone willing to buy something for a specific price. An ask is someone willing to sell something at a specific price. The bid-ask spread is the difference between the lowest ask and the highest bid. The point I am making is that the order books of markets don't accurately reflect the real underlying demand for the securities. Let's take a hedge fund, who would buy apple at $5. It's incredibly unlikely that they will actually place a bid on the order book at $5 for apple. Instead, they will monitor the price of apple and buy it if they see a good price. The value that human traders can provide is that they can tap into this hidden sources of liquidity that isn't reflected on the order book of the exchange. They can call up some manager and say "hey I can sell you something at a good price, you interested?" Without human traders, there is no way to tap into this liquidity. This occasionally causes problems in thinly traded markets or during after-hours trading.