Why do you need to give a patient a benzodiazipine prior to surgery with general anesthesia?

Helps the anesthetic. Maybe I should leave this for an anesthesiologist to answer, but basically it's because benzo's are relatively safe ; help relaxation ; forgetting the procedure, so that less general anesthesia (which is more hazardous) is needed. Often multiple drugs are used in anesthesia to balance the aims ; the medical risks.