Most people who live in a place with a variable climate have their homes insulated- except for their garages. Garages are usually the largest un-insulated spaces in the home, and they can use up a lot of energy in heating and air conditioning, especially if they share a wall with the main part of the house. Insulating this space saves energy, prevents loss of heat through the garage door, and ends up saving a lot of money.
Garage door insulation installation kits are usually offered at home improvement stores, and they are not very expensive. Definitely worth the effort and time, especially if the garage is used as a workspace or is otherwise inhabited.
The insulation methods vary, but achieve the same end result. There are some with foils that are reflective, containing two thick layers of super heavy duty polyethylene foam in between two thinner sheets of aluminum or other metalloid foil. These usually come out to about a fourth of an inch, sometimes a little more.
Other varieties of insulation are polystryene and various other foils, and some are easier to mount than others. Most of them can be installed by a professional garage door service person at fairly low cost.