It's nothing to do with knowing the trades 'in advance', they're the ones doing the trades! Market makers like Citadel are regulated to improve the prices offer

It's nothing to do with knowing the trades 'in advance', they're the ones doing the trades! Market makers like Citadel are regulated to improve the prices offered in the market place. A stock may trade for $50 on the exchange, but Citadel can sell it for $49.50, they must improve prices and they do. Part of the reason they can do this is that Robin Hood orders are 'safe'. If a huge hedge fund comes to Citadel and wants to trade a million shares in TSLA, they likely know something Citadel doesn't and will rip their face off. That's why Robin Hood orders are valuable, luckily, the incentives of the retail investor (buying a stock as cheap as possible) and the market maker (selling to someone who isn't about to pummel them) line up. If you think Citadel and its ilk are bad, you should see the guys they replaced! Market making is as old as the market itself, the modern advancements allow commission-free trading, massive liquidity and tiny spreads, before you'd be paying commission and buying a stock for dollars more than you could sell it for. They're good for the market and it's a shame this crazy conspiratorial thinking (which is never backed up by any real understanding) has taken hold of so many people.