The primary benefit was that it decreased transaction costs which is the reason spreads went down in the late 90s/early 00s when humans were still in the loop.

The primary benefit was that it decreased transaction costs which is the reason spreads went down in the late 90s/early 00s when humans were still in the loop. Humans are of course perfectly capable of rigging the market with or without computers, a fact so mind-blowingly obvious, I'm not sure why you'd accuse someone of believing that it's false. This particular thread was not about market rigging, it was about whether HFT can really be credited with decreasing spreads in the early 00s.