Predominantly two roles:
- US Dollar Clearing (Correspondent Banking Relationship or CBR as it is known)
- International Money Transfer to/from the United States.
In the first instance, the US Bank that are authorized to provide US Dollar Clearing, act as Correspondent Banks for other financial institutions worldwide for US Dollar Clearing.
As the US Dollar is printed by the US Government, the US licensed banks are the only banks that can do clearing for it. So when two banks in say Manila and Tokyo are settling in US Dollars, this transaction has to reflect through a ledger maintained by some US Bank (contrary to popular belief, the US Dollars in the banking system, all of it, the ledger entries are in the US with US banks and regional Federal Reserve Banks).
So, in plain English, if you have to clear a US Dollar transaction, it has to go through a US Bank.
In the second instance, if US Dollars need to be send abroad (either as US Dollars or as a foreign currency), a US bank will play the gateway role in it.
The same is true if US Dollars or another foreign currency is coming into the United States.