[ycombobreaker]

The technical aspects are more cross-cutting than your labels. "HFT" is for the most part just a latency threshold being crossed, whereas more functional roles such as market making, various forms of "pure" arbitrage, statistical arbitrage, prop trading, etc. have existed at the older latencies as well as the newer. All participants benefit from having lower-latency access than their competition, which creates a (thus far) neverending arms-race as firms leapfrog each other.