If you read Chair Gary Gensler testimony from May 6 2021 you can find the answer:

Why might brokerage firms want to increase customer trading? Who benefits from it?

In the last few years, most retail broker-dealers have stopped charging fees for trades. Instead, some make money through other streams, including a process called payment for order flow. Robinhood publicly reported $331 million in payment for order flow revenues in the first quarter of this year, more than triple the amount it brought in during the first quarter of 2020.

There are two kinds of payment for order flow I’d like to highlight: payments from wholesalers to brokers, and from exchanges to market makers and to brokers.

Here, I focus on payment from wholesalers to brokers. Here’s how that process works: retail broker-dealers enter into agreements with wholesalers, which purchase their order flow. Unlike public exchanges that must offer fair access to their publicly displayed quotes, these wholesalers can decide whether to execute these orders directly or to pass them along to be executed by the exchanges or other trading venues.

In addition, the wholesalers get valuable information from this order flow that other market participants get with a delay, if at all. In many aspects of the economy, from social media to search engines, access to data is a growing competitive advantage. Our capital markets are no different.

Higher volumes of trades generate more payments for order flow. This brings to mind a number of questions: Do broker-dealers have inherent conflicts of interest? If so, are customers getting best execution in the context of that conflict? Are broker-dealers incentivized to encourage customers to trade more frequently than is in those customers’ best interest? What are the policy implications with regard to the data aggregated by the purchasers of order flow?

These questions, while not new, were highlighted in the SEC’s recently settled enforcement action against Robinhood. As described in the Commission’s order, certain principal trading firms seeking to attract Robinhood’s order flow told them that there was a tradeoff between payment for order flow and price improvement for customers. Robinhood explicitly offered to accept less price improvement for its customers in exchange for receiving higher payment for order flow for itself. As a result, many Robinhood customers shouldered the costs of inferior executions; these costs might have exceeded any savings they might have thought they’d gotten from a zero commission.

Finally, it’s interesting to note that neither the United Kingdom nor Canada permits broker-dealers to route retail orders to off-exchange market makers in return for payments.

There are no free meals out there, check your best execution, it might not be as good as it needs to be.