The main technical reason is usually to change how out-of-focus light is rendered. In defocused areas, bright points and edges are effectively blurred by the shape of the aperture, so unusual iris shapes can change the look of bokeh and highlights.

A more rounded aperture tends to produce rounder blur discs. A star- or serrated shape can soften or alter the edges of those blur patterns, somewhat like a crude apodization effect, where the transition is less abrupt than a simple hard-edged polygon. That can change the appearance of out-of-focus outlines and specular highlights.

There can also be diffraction-related effects, since aperture shape influences diffraction behavior and resolving characteristics, but this is not generally a straightforward “better image quality” fix. In practice, these shapes are more often about rendering tradeoffs than a universal correction for lens defects.

In some designs, a non-round opening may also limit light in ways that better match the image area, but the most visible effect is usually the character of blur, not basic exposure or sharpness.

So: odd-shaped apertures are typically intentional optical design choices to influence bokeh/rendering, sometimes with mild apodization- or diffraction-related side effects, rather than simply being a mechanical quirk.