Your child needs a chickenpox vaccine because chickenpox can actually be a serious disease. In many cases, children experience a mild case of chickenpox, but other kids may have blisters that become infected. Others may develop pneumonia. There is no way to tell in advance the severity of the symptoms your child will experience.
Before vaccine was available, about 50 kids died every year from chickenpox, and about 1 in 500 kids who got chickenpox was hospitalized.