When you have intermediary trade business.
When you are required to open L/C by suppliers and do not have enough credit line of L/C at your bank.
Back-to-back L/C is a type of L/C issued in case of intermediary trade. Intermediate companies such as trading houses are sometimes required to open L/Cs by supplier and receive Export L/Cs from buyer. SMBC will issue a L/C for the intermediary company which is secured by the Export L/C (Master L/C). This L/C is called "Back-to-back L/C".