The front part of the eye is easy for the doctor to see. But many of the most important parts of the eye, such as the crystalline lens right behind the colored part of the eye or the retina at the back of the eye, are much harder to see. It is dark inside the eye, and the opening to the back of the eye — the black pupil — is very small. When we dilate the eye we make the pupil larger. This makes it much easier to see the critical parts behind the iris (the colored part of the eye). The crystalline lens is where cataracts form, and the retina in the back of the eye is where the light is collected and processed.
When we look at the back of the eye through a pupil that is not dilated, it is like looking into a room through a small keyhole. Dilate the eye, and it is like looking through an open door — a much better view!