The science on bicycle helmets has gotten pretty strong that they do minimize head injuries. They do reduce the risk of a serious head injury. At the same time, if you’re involved in a bicycle accident and you don’t have a helmet on, the question then becomes, did you suffer an injury that a helmet would have prevented. If you did not then it is of no consequence.
But if you did suffer a head injury, believe me that the other side, the insurance company, is going to argue that if you had one on, your injury might have been reduced and they’re going to hold you responsible or argue that your recovery should be reduced because of your own fault in not wearing a helmet. You want to eliminate that possibility for practical reasons and to protect your legal case at the same time.