What is meant/useful about transfer functions between 2 blocks in a block diagram?

The transfer functions are between two signals, not blocks. Usually blocks in a block diagram represent transfer functions. A direct link between two blocks means that the output of the first block (transfer function) serves as the input to the second one. Using the rules shown in class, one can combine different blocks (transfer functions) into a single block (transfer function), which makes it easier to reason about the system. '--Buzi 16:59, 3 November 2008 (PST)'