Why do you need the Arduino?
It certainly isn't the most robust application, but it is useful here.
The DSP I'm using has 16 presets, but I am only using 3 of them. If I were to use a rotary encoder to control it, I would have to cycle through all the presets sequentially--which I don't want. The Arduino allows me to call the 3 presets I'm wanting to use (and turn the reverb off) with a simple 4 position rotary switch. It also allows for the effects loop switching all in one shot. There are also some ground noise issues that the Arduino eliminates vs. an analog alternative.
It is entirely possible to do all the same things with analog circuitry, but using the Arduino makes it simpler, takes up less real estate, and requires less components.