In bright daylight, the scene contains a lot of light energy, and a well-illuminated scene does not need to be exposed through the lens element onto the sensor for very long to achieve the required exposure value.
In cloudy weather, the scene contains less light, and thus needs to be exposed for a longer period of time. Another way of achieving the correct exposure is to enlarge the iris opening, allowing more light to flow into the camera per unit of time.
Short exposure times limit motion blur.
A small iris opening increases the depth of field.