They provide 3rd party trust for electronic funds transfers between banks. Precisely what permissionless blockchains are doing without a 3rd party trust.
The nodes act as validators for transactions, inceasing speed and efficiency for settlement.
The Society for Worldwide Interbank Financial Telecommunication (SWIFT) provides secure electronic financial messaging services to financial institutions.
10 Million transactions per day transfer of nearly $5 trillion worldwide each day
Clearing House Interbank Payments System (CHIPS)
280,000 transactions per day with a total monetary value of $1.4 trillion
Federal Reserve Banks own and operate the Fedwire funds transfer system that serves as the primary domestic electronic funds transfer system in the United States
528,000 transactions valued at $2.1 trillion daily