HFT provides a great deal of liquidity and efficient pricing in markets that are adapted to it. It provides a real service, allowing people to transact without using a large bank or broker efficiently on an open market. The fact you can click buy and it buys on almost any stock is likely due to a HFT on the other side. That may not mean much to you directly but it does provide a lot of utility in markets. The biggest gripe people bring about HFT is it during high volatility HFTs usually pull out of the market at a time when people really value the increased liquidity. I think some of the more advanced HFT firms though have moved into longer time range trades which helps provide more liquidity in those markets.
Depends on their strategy but a lot of strategies depend on some sort of price discovery which requires having a probable estimate of what the current price should be and some form of market making around the level. In a very dislocated market the price is unknowable and it becomes gambling, and generally market making strategies are explicitly not about gambling but about facilitating trades around the “true” price.
