Pharmacists know all there is to know about prescription drugs - they are medication experts. The average pharmacist knows much more about drugs than the average physician - it simply is not a physician's field.
There is a lot more to a pharmacist's career than just printing labels and counting pills. Because everyone can make a mistake, a pharmacist will double check and make sure that the medication a physician has prescribed to a patient will not interact with other drugs the patient may be taking. It is not unusual for a pharmacist to call the physician to confirm dosage, as it is easy to write the wrong amount, which will result in a dosage that is either too low or too high. They will make dose adjustments, they will check chemotherapy calculations - they will basically act as a gatekeeper between the patient and harmful drug interactions.
There are also many time-consuming duties that a pharmacist deals with, such as ordering supplies, figuring things out with insurance companies, and handling the many regulatory compliance issues. Physicians are glad there is a division of labour so that they can concentrate on treating their patients.