The rising margin between what banks pay depositors and what they charge borrowers when they lend out those deposits translates into higher profits.

The banks will not immediately raise the rates they pay depositors, and then not by the full amount of their higher charges for borrowers.

Ultra-low interest rates have crushed net interest margins, the difference between the rate the banks charge borrowers and pay depositors.

This creates a fundamental fragility, as a bank's assets can not be liquidated in the event of a crisis to pay all depositors.

Such insurance pays depositors all or part of their losses in the case of a bank run.