When doing banking, the banks and its managers have also a liability related with the bank account of clients acitivites. Therefore, most international banks ask for reference letter from the original previous bank and doing KYC/AML procedure.
But most banks, especially in Germany, doesn't provide the reference letter, so the banks force the clients "to stick as customer" with its bank, especially the german government want to spy on the clients activities.
Therefore I personally have solved it without the need of such thing as a reference letter and know many solutions to it.