High Frequency Trading is a technological progression which effectively now dominates the short term trading niche once occupied by scalp traders. Back in the day before the internet and electronic trading, human traders on the floor would often be buying and selling within seconds, as fast as they could shout the order. This was how, from the beginning of modern day financial markets over 100 years ago, liquidity and price discovery happened. Often a trader wouldn't really know much more about what they were trading than it's name.
Also, it's important to remember that HFT's trade against one another, the markets in essence match buyers and sellers. There are arguments to be made about aspects of high frequency trading, just as with anything else in life, but hey are not simple, black and white ones.