HFT is a different thing from what is being discussed in this thread. With HFT you're talking custom ASICs running within light-nanoseconds range of the target exchange. Ocaml very much isn't in this picture. This is about human-speed trading. Which also provides liquidity and correction of instrument prices towards their fair value, just at a different level.
The societal value of either is debatable all the same, mind you. It's more that wherever you have markets, you have money-making opportunities that can be leveraged, and therefore are.
There’s a few orders of magnitude between “human-speed trading” and the absolute bleeding edge of HFT. A company like Jane Street still does automated trading far faster than any human could.
Sure, Jane Street probably isn’t the fastest in the business, but I wouldn’t be surprised if they’ve got FPGAs or ASICs, dedicated high speed pipes to shave off milliseconds of latency, things like that.
In turn that leads to more efficient markets since prices converge to their "correct" value faster.