Partly the additional sizing information required for contacts (Diameter and curvature of the cornea are needed for contact lenses but not for glasses) and the power also changes because the physical distance between the eye and the lens is different.
Here is a page explaining the differences between the two prescription types with explanation ...
The value of dilation of the pupil far outweighs the inconvenience.In fact, I'd offer that if an exam is done without it, you potentially lose the following:
--full access to all the structure of the retina, macula, and associated blood vessels. Dilation opens up the aperture of the pupil enough for the examiner to see all there is to see. I could fill up ...
The 20:S notation means that a person with normal vision standing S feet away can see that line on the Snellen chart clearly, but fails to see smaller lines.
Size of Letters Required
for Visibility as a
Function of Viewing
Distance and Observer
3.1 MEANING OF THE SNELLEN NOTATION (page 12)
At 20 feet (6 metres), this person with normal vision ...