Leasing of aircraft has many advantages to the operators, and good income for banks.
Operators are able to deduct leasing costs from their revenue, reducing their income tax liabilities. They also need not worry about how to dispose the aircraft and storage costs once the aircraft is not needed for business operations, they just simply return the aircraft to the bank.
As for the bank, they put up the capital sum of money for the purchase, and charge extra sum to the aircraft operators, not unlike mortgage interests of your cars. Profit return is attractive. The bank retain owenrship of aircraft, and should the aircraft be returned, they can lease it to another airlines operator.