" CitSec’s market-making business also entails the somewhat controversial business of payment for order flow, much scrutinized during the GameStop imbroglio in 2021, where Citadel pays retail brokerage firms like Robinhood Markets and Charles Schwab to process their trading orders. In the first quarter of 2025 alone, Citadel paid brokers $388 million—much of that for options contracts, according to Global Trading—more than any other market maker. Citadel says it gets the most orders because it has the best execution, and that the money it pays to the brokers is essentially rebating back part of the spread. Those payments allow brokers to offer rock-bottom-priced or even free trading to investors—which begets more trading, which benefits Citadel."

WHY would they pay for order flow?