An additional effect of having the penis held upwards is that the scrotum is fully exposed and displayed.

The ancient Greeks were uninhibited in their aesthetics.  For them an aesthetic foreskin covered the glans completely and extended beyond to form a long tapered tip.  An exposed glans was treated with disdain.  Komasts (party revellers) also wore the kynodesme and its use may have been widespread especially amongst those who had short foreskins that were retracted when their penises were flacid and exposed the glans penis.

Continuous wearing of the kynodesme may have the effect of stretching and elongating the foreskin and thus making it more aesthetically pleasing.

This suggests that Greek males went naked frequently or, at least, what clothing they did wear was not worn to hide the genitals.  If they are going to such trouble to ensure the aesthetic presentation of their genitalia it is unlikely they would consciously hide them.

In Amazonia, Amerindian tribesmen are naked all the time except for a cord which they tie firmly round the prepuce and round the waist. Like the ancient Greek wearers of the kynodesme the penis is held upright and the scrotum fully exposed and displayed.  Many of the Amazon tribes have now been westernised and wear pants rather than penis cords; it can't be long before another aspect of nude culture is consigned to the archive. ... or can it?