Mushan, Fred has explained to you why the anode is polarized. I would like to make addition to explain why the anodes cannot be polarized in some cases while insoluble anodes are used in other cases. If anodes act as the source of a plating substance, such as in copper cyanide baths, the anodes must be kept in an active dissolution that also balances the metal concentration in the plating solution. In the cases that insoluble anodes are used in plating, the metal salts are added to the plating solutions in accordance with the metal ion consumption. The anodes are only designed to complete the electrolytic circuit.
Anodic reaction is mainly oxygen evolution in most cases.
Anodes must be kept in passive state otherwise dissolved products from anodes may contaminate the plating baths. The insoluble anodes often have catalytic effect on the anodic reaction. A plating system determines what kind of anodes should be used. The anodes must then work properly and consistently under the condition required by the plating system. I want to post a question or inquiry of my own.