Why do periwinkles need shells when the tide goes out?

Periwinkles need shells to protect themselves from desiccation and predators when the tide goes out. The shell acts as a barrier against the harsh environmental conditions of being exposed to air and sunlight, helping to retain moisture. Additionally, the shell provides physical protection from potential threats, such as larger animals. This adaptation is crucial for their survival in intertidal zones where they experience fluctuating conditions.