Building code for garages do not require that the garage is insulated.  It is a personal choice.

To insulate a garage, two areas much be insulated.

First, a material that slows the transference of the temperature outside to the interior of the garage is place in the space between the exterior wall and the interior drywall.

Secondly, if you want to truly insulate your garage, the material must be placed on the back of the garage door itself.

Insulation will then regulate the temperature inside and work as a sound barrier as well.

Pros of Insulating Your Garage

The positives of insulating your garage include the energy-efficiently, especially if you plan to use your garage space for other purposes.  For example, if you want to use your garage for a work bench or exercise area, and you want to heat or cool the area depending on the season, insulation will certainly save on your energy bill.

Temperature regulation can also allow you to store more items in the garage that are heat sensitive.  These include artwork, paints, candles and other environmentally sensitive materials.

The insulation will also provide a sound barrier for your neighbors if you play music or work on a motor vehicle.

Cons of Insulating Your Garage

There is a cost to insulating your garage.  It makes no sense to insulate the walls of the garage if you do not insulate the garage door itself.  The garage door will be a huge source of loss of heat or cooling.

In addition, if you live in an area that reaches high temperatures during the summer, you may have the opposite result of what you intended.  That high heat can be trapped into your garage, and a running or hot car that is parked in your garage can make the garage that much hotter.  The heat will have nowhere to go, and you will have a garage that is hotter than outside.

Summary

The decision to insulate your garage depends on the uses of your garage and the environment you live in.  Are you in a very hot summer area?  Or do you live in a predominantly cold climate where you use your garage for other purposes?  These are questions you can answer in your decision making.