The cod-piece on plate armour mirrored contemporary fashion. It wasn't that it was required for protection so much as a way to boast. Who'd have thought eh?
For anyone interested, the codpiece is thought to have originated as a way for men to remain 'decent' as fashion dictated skin-tight hose for the wealthy, and looser-fitting two-part long pants or breeches for the common man. When the hose became supplemented by the short puffy trousers so familiar in late Medieval/Early Modern paintings, the codpiece was retained. Needless to say, most were not filled to capacity.