In the summer, when the ground and air temperatures are comparatively warm, there is no great need for insulation underneath you.

When the ground is cold or the air temperature is cold, you do need effective insulation beneath you to stay warm. Most sleeping bags do not do a good job of insulating beneath you because the insulation, whether goose down or synthetic, is compacted by your weight and loses effectiveness. A warmer rated bag will help very little to insulate you from the cold below.

Various foam pads or insulated air pads are necessary in cold weather. Check out their 'R' rating to discover how well they insulate.