When people are told they need a bite guard it is because there are signs and/or symptoms that they grind or clench their teeth.
Grinding, or bruxism, is the sliding of the teeth together and clenching is the squeezing of the teeth tightly together. These habits can occur due to stress or occlusal discrepancies known as a bad bite. Most people are not even aware that it is happening because it can happen while sleeping.
Signs of bruxism include flattening of the normal contours of the teeth and abfractions, which are the small ditches in the tooth at the gumline. Other symptoms include headaches, pain in the muscles of the head, neck and jaw, and popping or clicking of the jaw joint.
Having a proper, custom-made bite guard takes the forces off the teeth, muscles and joints, making the process nondestructive. The drug store versions will not do this. The earlier the habit is caught, the less damage to the teeth, bone, muscles and joints.