Short story is, they're making money you don't have access to. Exchanges add fees to every trade that goes through an exchange. If a stock costs $10 with a $1 fee, and someone's willing to sell it to you for $10.50 with a $0 fee, wouldn't you take it? That's fifty cents of price improvement over national best offer. So Citadel batches all the orders for that stock and does one big trade (per stock) at the end of the day, and everything settles out -- you got cheaper stock, they made some money for their trouble. Enough money to kick back $0.20 or so to Robinhood for referring you as a client.