Financial institutions such as a bank monetize a portion of the total funds (in this case airbnb) held in their accounts to lend out and collect interest on or to invest. Banks often purchase marketable securities to hold in their portfolios; these are usually one of two main sources of revenue, along with loans. Investment securities held by banks as collateral can take the form of equity (ownership stakes) in corporations or debt securities. That's how banks make money. In this case they just happen to double as a payment service. How do you think they could afford to do it for free until now?