High-frequency trading generally has a large affect on the volume of trades occurring. The more orders in the market, the more people who can buy. Large volumes of trades entering the market also mean that the prices are updated regularly, meaning valuations are more accurate. Having said that, both of the above are a bad thing when something goes wrong. If a bad price enters the market, the volume of trades done on that bad price are huge. This can have a very large affect on the market.