So I'm not an expert either, but financial books are some of my favorite. I think the easy take away is that the bid ask spread is the transaction cost. If you

So I'm not an expert either, but financial books are some of my favorite. I think the easy take away is that the bid ask spread is the transaction cost. If you can buy and sell a stock for the same price (minus price movements), it's very efficient to move your money in and out of different assets. With the bid-ask spread being wider, that adds a transaction cost to every trade, as you can't sell for the price you just bought. This means that trades are more expensive and riskier. HFT provides liquidity that reduces those risks. HFT seems to me to be one of those words that means two different things and people select the version of HFT that fits their arguments. "Evil" HFT is a rent seeker that just can get in front of the trades you make without adding value, you're just paying an extra penny or something for transaction. "Good" HFT is a market maker with tons of info who just gets there first every time because they're FAST. You can see how the amount of info HFT turns it "good" or "bad", and that's a tightly held trade secret for most of the trading firms, so it's a grey area.