What is the main function of the pupil of the eye?

The pupil serves two important functions - to control the illumination in the eye, and to improve the optical quality of the image on the retina. The first function is obvious - the smaller the hole, the less light gets in. It therefore serves to regulate the light level within the retina. The second function is very important because as the pupil gets larger, more aberrations are created and the image on the retina gets "messier". People with no iris (aniridia) have very poor visual acuity because the eye can't get a sharp image, both because there is to much light (over-saturation) and because there are lots of optical aberrations present. One final function of the pupil is to increase depth of focus, similar to an aperture stop on a camera. This basically increases the range (distance) over which things appear sharp.