A spectator ion is used, because it just "spectates". It does not get involved in the reaction. Spectator ions can be seen in a net ionic equation, but they are canceled out because they do not...
spectator ions are the ions that do not take part in the chemical reaction. They are usually ions that form soluble compounds. Spectator ions often have a single charge and a large radius.
Spectator ions, in a chemical reaction, are ions that don't take part in the overall reaction. For instance, consider the following reaction: Pb2+ (aq) + 2 NO3 (aq) + 2 K+(aq) + 2 I-(aq) --->...