I want to guess that most of the shrimp Jan is using will go in the sauce. So it will be shrimp with shrimp sauce to make sure the shrimp doesn't lose its flavour in the sauce and other seasonings. Typical in a lot of cooking manga. It's like when you freeze drinks and then use that as the ice cubes so as to not dilute the taste of the original drink (because if you'd used regular ice, it'd be like adding water to your drink). So frozen coffee for ice coffee, frozen apple juice for apple juice, etc.