However, pouring unspeakable amounts of money, watts and person-hours into completely unnecessary and wasteful infrastructure just because emergent properties of a system mean that billion dollar companies and their owners can make a lot of money by being a picosecond faster than each other in an eternal battle is of extremely questionable utility.
What exactly do we as a society get in exchange?
Arguing about HFT feels like you're questioning people about why they're spending billions on excavating single, individual sand grains and the defense you get is "well, sand is an incredibly important building material. It is used in concrete, and we wouldn't have modern society without it". Sure that's true, but why is it that nobody can tell me how what you're doing is actually necessary for that.
Does that require HFT, or just T?
That's the theory, anyways.
No.
Market event A is seen to correlate strongly with market event B. The correlation is so obvious that multiple participants compete to make the B trade first in reaction to A. It's part of having a healthy market, electronic or otherwise.