Can somebody please explain about optical cables please? Ref Samsung home cinema system?

Hi, just bought Samsung HT-Z320R 5.1 home cinema system. Shop lady sold me an optical cable. Got it home and the optical cable just won't go in the dvd player or my telly. My hubby just tried it anyway with the HDMI lead alone. It works fine. Why would we need an optical cable? Is it necessary or have I been mis-sold it? Kind regards, Jen