Buffers imply interrupt driven serial communication.

The point of input buffer is, that the "main program" might be busy with something while the interrupt keeps receiving the bytes and stores them into a temporary storage (a FIFO = circular buffer). The main program can "pick up" the data from the buffer later.

The point of output buffer is something similar, the main program may "spit out" a load of bytes at once into the buffer, and the interrupt takes care of transmitting it gradually, while the "main" does already something else.

Jan Waclawek