Why is this?

[VCE123456789]

The electrode for the I2(aq)/I-(aq) half-cell would need to be a non-reactive electrode, either platinum or graphite (C). Why is a platinum elcetrode required?

[masonnnn]

if it was a copper electrode for instance, it would change the reaction that is occurring resulting in the oxidation of copper.
in most instances you'd want to use graphite or platinum as they won't react or interfere with the galvanic process required.