Take any effect or chain of effect (even your whole board) and run them through the send and return of the Blender. As you turn the knob you blend in your original clean tone with the effected signal. Simple and effective!
This can also be used as a tuner out or simple effects loop to activate a string of multiple pedals for a preset tone. Note that this is a buffered circuit and not just a simple passive looper like many out there.