https://www.technologyreview.com/s/603431/as-goldman-embrace...
It makes the markets more efficient which is good for everyone.
Do you think there was any benefit from having humans do this work? Was there any subjective analysis used, or were these 600 people just human calculators?
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.
Now as everything has become more efficient, humans are replaced by algos, which are way cheaper. In turn, they also require less for upkeep, and so charge less in edge, which means you have better bid/ask prices, lower trading fees, etc.