Bare spots should repaired to prevent more damage. Bare spots start out in usually a small area, but with continued traffic, will increase the size and add more to repair. Bare spots in grass can be caused from pets, children playing or from heavy foot traffic. In some cases insects may be the culprit.
Insects can cause thinning or dying grass.
The method of repair depends on the cause. If insects are the problem you will need to spray the area with an insecticide before trying to grow new grass. High traffic areas, may present an ongoing problem. You may consider placing stepping stones to walk on, or a gravel pathway, or a barrier. When you decide to repair, early spring is the best time.