Where do you use BDC?

• transferring data from another system when you install your SAP System
• regularly transferring data that is captured by a non-SAP system in your company into the SAP System. Assume, for example, that data collection in some areas of your company is still performed by a non-SAP system. You can still consolidate all of your data in the SAP System by exporting the data from the other system and reading it into the SAP System with batch input.

You can also use batch input to transfer data between two R/3 Systems. However, there are more direct methods for doing this, such as RFC (remote function calls).