Not entirely "nothing", as a result of HFT market making all participants are getting smaller spreads and increased liquidity. However... That has huge diminishing returns. The first few steps to electronic markets and fast market makers dropped the spread considerably (making trading cheaper for everyone), all the steps that came after cost a lot more and didn't really improve things much.

So they're not adding a lot now relative to the cost and complexity. But banning the whole practice will make the markets function much worse and more expensive for everyone.