For any electrochemical experiment you need a standard. The classical standard for electrochemical potentials is H2 = 2 H+ + 2 e-. If this is not applicable in the experiment, people use other standards. Ferrocene is very suitable in non-aqueous solutions.
The benefin of ferrocene is that its oxidation wave is frequently reversible, however, depending on the solvent this might not be the case (for details you can look up the work of William Geiger).