The R-value of a garage door tells you how well it insulates the garage. You might think it's not important to have a garage door that insulates but there are several reason you might want to consider this. The biggest reason to use an insulated door on your garage is to keep from losing heat from the home, if your garage is attached you are likely loosing a lot of heat from the garage. Also if you spend a lot of time in the garage, say you have a workshop in it, you'll want to keep the heat that you have inside to keep warm while you're working. Last there is the fact that an insulated panel is built sturdier than a non-insulated. Meaning you'll find a door with a higher R-Value will take a hit better than one with no R-Value. Check the R-Value of the door before you buy it, consider where the door is being installed, and what the garage is used for.