The temperature of the attic primarily controls the temperature of the entire home. Lighter, warmer air rises causing the attic to heat up quicker than the rest of the home. Heat flows through the home from warmer to cooler until there is no longer a temperature difference. If the attic is too hot the rest of the home will have a harder time cooling down. Insulation materials work by slowing that heat flow. This allows the AC to work less to cool the home and your energy usage to lessen.
Outside air enters and conditioned air leaves your house uncontrollably through cracks and openings throughout your home. According to energystar.gov, if you added up all the leaks, holes and gaps in a typical home's envelope, it would be the equivalent of having a window open every day of the year! Sealing your home along with the right amount of insulation, can make a real difference on your utility bill.