Benefits of a Sleeping Pad
The primary benefits of a sleeping pad are insulation and comfort. Sleeping pads are required for cold weather camping. Your body weight compresses the section of your sleeping bag
between you and the ground, removing most of your sleeping bag's loft, and thus its insulation (in this area). Therefore if you sleep directly on the ground you will loose a large amount of body heat. In fact, a sleeping bag's rating is determined while using the equivalent of a Therm-a-rest trail pad. If you don't have a pad that provides a similar amount of insulation, you should expect your bag to not live up to the temperature rating that the manufacturer gives it. Without a pad, you should expect to whittle away at least 15 degrees off its rating.
Sleeping pads provide insulation in the zones where your sleeping bag
will not. 2/3 length sleeping pads are also popular because they reduce weight and still provide insulation to the primary body heat loss areas.Full Article