A garage door opening is a very large hole in your garage. Insulated doors will help to reduce the amount of outside temperatures that enters your garage, thus keeping it more comfortable for you. This is important for a number of reasons:
If your garage is attached to your home, any air in the garage can travel through the doorway to your living area.
If you use your garage as a workshop, your comfort will be a top priority.
If your garage is below another room in your home, air can travel through the ceiling of the garage into the floor of the room above. An insulated door will keep temperatures in the garage fairly stable to reduce the temperature fluctuation in the room.
An insulated door is generally quieter and has a more attractive interior than an uninsulated door.