As a young coder, I played around with Arduino, but I never realized that Arduino could interface with Processing. It really is a pleasant surprise to see something that we are learning today has so many uses! With Arduino, the circuit board can “talk” to the computer and communicate information with each other. Arduino uses Processing because it is a easy way to send code to other computers. I guess because of its simplicity and intuitive design, it makes it easier to transmit Processing code over Java (which requires compilers and what not).
Read more about interfacing here!