My experience is limited, so take with a grain of salt: my sense so far is that it helps stamp out any statistical/numerical inefficiencies in the markets that humans would otherwise be inefficient/slow to adjust to. For instance, if a straight up arbitrage opportunity exists across exchanges, automated strategies ensure that it goes away very quickly, much more quickly than discretionary human traders would. If there are proven statistical trends to the markets, then those should be acted on as well, because it means we'll arrive at the fair price of an asset sooner rather than later. A simple example: if a stock closely tracks the price of oil, and there's a quick uptick in the price of oil, it's better for the price of the stock to get immediately adjusted upwards rather than after a delay (and purely quantitative, automated strategies make this happen). I think the name of the game is that you have a bunch of people acting in their best interest, and thereby giving us an efficient market, which benefits the rest of the world by ensuring low slippage, fair prices, high liquidity, etc.