This project was started by johndavid400 whose intention was to get more I/O pins from Arduino. So why not to add another Arduino board on top and almost double the number of pins. As result you can have 12PWM pins, 12 Analog, 40 Digital pins and 4 external interrupts.
Actually you don’t have to build a now complete Arduino board, as stacking them allows to exclude power supply and USB interface. both Arduino’s communicate via I2C interface this way consuming only two digital pins, while other may be used for other purposes. To make things work, there has to be sketches for both arduino – master and slave. For now, it only has sample sketch for controlling digital pins. But after some coding it can be converted in to full functioning expander or who knows maybe some dual core routines. If you need more I/O’s fro Arduino, this is right choice.