atari_lib.py defining the neural network

Hello! I am very interesting in looking into the architecture of the DQNs. I opened up the file the Dopamine group kindly provided in Discrete_Domains --> atari_lib.py which defines the neural networks from the original Nature paper.

I was wondering why-- for each neural architecture -- _**two**_ neural equivalent neural nets, one set as a definition and the other set as a class, are provided (see image below)? What would happen if one had just defined the class, for instance? I apologize if this is a very noob question, but thank you very much in advance for the insight!