Shadyshackle

So I'm a bit new to this modding thing and after looking as Psi's source code, I had a thought. Psi only uses a common proxy and the subclass client proxy. This is different from most tutorials on the subject where you have both a client proxy subclass and a server proxy subclass. From my understanding, there's two instances where a mod could be run: on a server(which only has server code) and a Combined client(which has both client and server code). Therefore wouldn't you just need a common proxy when the mod is being run a server and a client proxy that simply overrides the common proxy to add things like gui implementation etc? Where would you ever have code that would be run on the server proxy but not on the combined client, as the combined client has all the code of the server and more?