During a spontaneous electrochemical reaction, electrons flow from high potential energy to low potential energy. This change in potential energy is just the free energy change for the reaction. Since potential energy is proportional to the the charge of the electron, the free energy change is proportional to the charge AND number of electrons transferred.
E o is the potential energy change PER coulomb of charge transferred.
Galvanic cells use spontaneous chemical reactions to provide useful electrical work. Therefore, the electrons that are transferred during the reaction must be forced along an external cable to provide a current. The external wire connecting the two electrodes allows electrons to flow and creates a device which can provide electrical work. This model cell will not be very effective because the resistance is too great.