This isn’t an easy way to make money. It is regulatorily complex to run a trading desk, and it can consume a lot of capital. But it was a significant revenue generator. Autonomous Research analyst Christian Bolu has noted that E*Trade’s revenue-per-trade was roughly 40% lower after it sold its market-making unit. Likewise, based on what Interactive Brokers historically generated in typical revenue as a market maker, Robinhood could in theory have generated even more on its volume than what it does today, according to analyst Steven Chubak of Wolfe Research.

https://www.wsj.com/articles/why-free-trading-will-never-be-free-11631871038