One of the most valuable aspects of using an automated system is its ability to operate at a pace no human could. High-frequency trading (or scalping) is the method of using an automated system to potentially make hundreds of thousands of trades in a fraction of a second.
The biggest criticism of HFT is that it allows the big players to dominate because they can trade in such substantial blocks (using algorithms). This type of trading adds liquidity to the market; however, many consider this a negative because the liquidity produced only lasts for seconds – too short a time for non-HFT traders to benefit from it.
A positive of HFT is that it removes small bid-ask spreads. A bid-ask spread or a bid-offer spread is the difference between the price at which an asset can be sold, versus the price at which the same asset can be bought. By widening the gap between, HFT traders can often exploit the system and make more money.