Imagine a market that traded every hour. So everyone lined up their orders and what they were selling and at the top of the hour all the trades were processed.

Imagine a market that traded every hour. So everyone lined up their orders and what they were selling and at the top of the hour all the trades were processed. Now imagine this on 9/11, or a fed release. Prices from an hour ago are just really not enough information to understand what the prices should be at now, so it ends up being very confusing (and not to mention bursty!). You could put in an order only to realize your price was too low, and you don't get filled, and have to wait another whole hour! Tons of inefficiencies there. Those same inefficiencies in the market would be seen at 59 minutes, at 30 minutes, at 10 minutes, but get less and less impactful each time. HFTs basically provide a service that gives you almost instant liquidity and market making, which is valuable to anyone who's looking to act quickly buying or selling.