Theoretically, it increases liquidity - the chances of a trade executing, and it keeps the trading price of the traded stocks close to their real value, whateve

Theoretically, it increases liquidity - the chances of a trade executing, and it keeps the trading price of the traded stocks close to their real value, whatever that is. In practice, it's probably fairly neutral, so long as no-one is playing silly b@#$%^rs, front-running other people's trades or similar.