In order to create ions, electrons have to be carried from the one side of the system to the other. The electrons are split off from the fuel by the metal plate at the fuel side and need to travel to the air side to complete the reaction. Because the screen doesn't let electrons pass through, they go through a separate wire, to the other metal plate on the air side. The travel of electrons creates electric current (electricity). The wire is where electricity can be used. For example, the wire can be cut in half, and a light bulb can be connected in between the two halves.