[pi-rat]

Generally gets attributed with:

- Increased liquidity. Ensures there's actually something to be traded available globally, and swiftly moves it to places where it's lacking.

- Tighter spreads, the difference between you buying and then selling again is lower. Which often is good for the "actual users" of the market.

- Global prices / less geographical differences in prices. Generally you can trust you get the right price no matter what venue you trade at, as any arbitrage opportunity has likely already been executed on.

- etc..

[munk-a]

> Tighter spreads, the difference between you buying and then selling again is lower. Which often is good for the "actual users" of the market.

I just wanted to highlight this one in particular - the spread is tighter because HFTs eat the spread and reduce the error that market players can benefit from. The spread is disappearing because of rent-seeking from the HFTs.

[yxhuvud]

What needs to be pointed out is that the rent and spread is the same thing in this equation. Before the rise of HFT actual people performed these functions, and then the spread/rent-seeking was a lot higher.