What is the role of steganography in network security? Quick definition:   


  Hiding secret messages inside what seem to be harmless messages is nothing new. The word steganography itself originated in Greece and means “covered writing”. During important historic events of our past, steganography was often used to trade personal secrets, plan covert operations and send political espionage information.


source: steganography

What is role of steganography in network security?
I’m not aware of any uses of steganography in network applications, and considering almost all of the networking protocols are open protocols, there’s little room for the application of it. If you need security, you use various forms of encryption.

That being said, the only example I can think of of steganography in modern days is, some printers also printing out some form of a serial code and timestamp in a (almost) hidden part of the printed out material.