High frequency trading presents a lot of interesting puzzles. The Booth faculty lunchroom has hosted some interesting discussions: "what possible social use is it to have price discovery in a microsecond instead of a millisecond?" "I don’t know, but there’s a theorem that says if it’s profitable it’s socially beneficial." "Not if there are externalities" "Ok, where’s the externality?" At which point we all agree we don’t know what the heck is going on.
Tragedy of the Commons.
There is valuable unowned information in the pattern of trades and prices. This is an arms race to exploit it.