Return leaks can cause your coil to act like a filter.  With small rips, separations or cracks in your return ducting, air can bypass your filter, enter your furnace, and begin the deterioration process on your furnace or air handler.  This includes the plugging of your indoor coil, which decreases the efficiency of your cooling system and drastically cuts the air supply to the rest of your home or business.  The polluted air then enters your supply duct system, and is then distributed into your home or business.  Sealing the return side is just as important, if not more, than your supply ducts.  This can improve your allergies along with other indoor symptoms.

Indoor air is found to be up to 40 times more polluted than outdoor air.
Most people spend 80-90% of their time indoors.
50% of all illness is caused or aggravated by polluted indoor air.
The average person breathes 500 cubic feet of air daily.
There are up to 30 million dust and pollutant particles in one cubic foot of air.
The average home accumulates over 40 pounds of dust a year.