There are at least 2 benefits:

* They are always willing to trade, so if someone wants to sell/buy a stock, they can get it from an HFT rather than needing to wait for someone who wants it long-term. (The HFT aims to sell it later to someone who wants it, they don't want to keep it).

This is called 'providing liquidity'

* They ensure prices remain 'in sync'. If prices don't match in some sense, then people need to worry about where to get a stock, rather than just what stock to get. The same goes for e.g. keeping stocks in sync with options. This way, someone who buys a stock gets an accurate price.

This is called 'arbitrage'

There is more to these concepts, especially arbitrage. But these are the core benefits of HFT.