"Why does the veterinarian need to see my dog/cat?  Can they just prescribe something over the phone?"

The Veterinary Practice Act requires that a veterinarian examine patients on a regular basis in order to prescribe medications (including over the counter medication). 

A comprehensive examination consists of checking eyes, ears, teeth, respiratory health, cardiac health, temperature, coat, skin, musculature, skeletal condition, and discussing everyday habits, behavior, and nutrition with the owner. While your veterinarian is discussing Fluffy's strange eating habits she/he is also checking for subtle signs of illness.  Many pet owners overlook the importance of an annual exam (especially in cats).  Animals will hide their illness until the progression has reached the point of irreversible damage (e.g. increased drinking could be the start of kidney failure or diabetes).

For more information on Veterinary-Client-Patient-Relationships (or VCPRs) please read the following from the American Veterinary Medical Association: