In reality, the Arduino just adds unnecessary complexity to the design. The control system could have been implemented using only the Raspberry PI because the BrewPI only uses digital I/O. My guess is that the project started out as an Arduino project with the Raspberry PI being grafted on at a latter date.
With that said, I laud the effort that the designer put into bringing this product to the market. There's a lot that goes into bringing a product to market that's above and beyond the basic software and hardware engineering work. For example, the case in which the unit resides did not design itself, and the documentation and website did not magically appear out of thin air.