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Imagine you're a market maker. You make $ on the bid/ask spread - giving liquidity to traders by buying low & selling high millions of times per day. As long as you can identify profitable trades, the more volume you get the better.

In exchange for skimming $ from the spread, you take on some risk of the market moving against you. If an illiquid stock tanks while you're making a market for it, you end up buying a falling stock with no one to sell it to. That's a no no.

You're a good market maker. You know how to avoid being a bag-holder. You stick to large-cap, liquid markets to help reduce the risk that no one shows up to offload your inventory.

Your margins get so good that you're willing to pay brokers to give you trades that fit your strategy. Instead of drumming up interest in your market yourself, you give some $ to brokers to do it for you. That’s payment for order flow (PFOF).