The difference between bid and ask is the profit for the dealer and the bid-ask spread is the profit for the dealer.

中间商的利润来自出价和要价的差别,差价就是中间商的利润

耶鲁公开课 - 金融市场课程节选

The dealer will charge a higher price for you to buy the furniture than will give to you to purchase the furniture from you; that's how the dealer makes a difference.

中间商给你的报价,会比向你收购同样家具的开价高,这就是中间商如何赚取差价

耶鲁公开课 - 金融市场课程节选