You might want to block input data if the external procedure needs to alternate reading from input groups. Without the blocking functionality, you would need to write the procedure code to buffer incoming data. You can block input data instead of buffering it which usually increases session performance.
For example, you need to create an external procedure with two input groups. The external procedure reads a row from the first input group and then reads a row from the second input group. If you use blocking, you can write the external procedure code to block the flow of data from one input group while it processes the data from the other input group. When you write the external procedure code to block data, you increase performance because the procedure does not need to copy the source data to a buffer. However, you could write the external procedure to allocate a buffer and copy the data from one input group to the buffer until it is ready to process the data. Copying source data to a buffer decreases performance.