Your pharmacy vendor is probably Caremark. They own CVS. Just guessing, but you medical insurance only processes medical (doctor, hospital, physical therapy, etc.) claims. They have a company like Caremark to process the pharmacy claims.

RJaye is correct, the insurance company works a contractual deal with Caremark/CVS to pay less for a major portion of your prescriptions. The idea being if the insurance company saves money, they don't have to increase your premiums.

There's also a cost savings when the vendor (Caremark) just happens to own the pharmacy where you need do business.

It's my understanding that you may be able to use a pharmacy other than CVS, but you may pay more out of pocket.