High frequency traders are simply are able to provide the most competitive price in the market. They usually do this with a strong hunch that they will be able to get out of that position in a short period of time at a better price. Are they providing a service? Absolutely. When HF traders are involved in a transaction, risk is transferred to them. No matter how short the interval is that they're trading on, they're still taking on risk. The party trading against them doesn't have to worry about what way the market is going to move after the trade is complete and they are willing to pay for that.