Why does the doctor sync with the R waves for sync cardioversion?

Safety. An electrical shock at the wrong time during the cardiac cycle can unduce ventricular fibrillation, a potentially life threatening dysrhythmia. Syncing the timing of the electrical discharge helps prevent this from occurring.
Prevent. To prevent dangerous arrhythmia called ventricular fibrillation which can be induce if the the shock is not correctly synced with cardiac cycle r and t waves.