Regarding international trade, why do two trading banks in different countries use a 3rd bank to handle transactions?

Most U.S. exporters employ letters of credit (LCs) and documentary collections (DCs), the two most important trade finance tools and effect transactions through the international organization known as Society for Worldwide Inter-bank Financial Telecommunication (SWIFT). This organization is the global provider of secure financial messaging services.
Typically, international trade is conducted by two but many a time by three banks for risk minimization of the international transaction.
Issuing Bank
The Uniform Customs and Practice for Documentary Credits allow the issuing bank a reasonable amount of time after receipt of the documents to examine them and to honor the letter of credit by making the specified payment to the beneficiary. Then the issuing bank completes the transaction by receiving reimbursement from the bank customer for whom the letter of credit was issued.
Advising Bank
Also, it is the responsibility of the advising bank to make sure that the appropriate documents are collected and sent to the issuing bank.
Confirming Bank
Usually, the confirming bank is also the advising bank, but this is not a requirement, and the functions can be separate. To cover the risk of the issuing bank not paying, an exporter may have a bank in its own country confirm the letter of credit, in which case the confirming bank agrees to pay the exporter if the issuing bank defaults.