Granular Activated Carbon, is primarily used to remove chlorine from your water supply.   Chlorine is used by water treatment facilities to kill bacteria in the water supply.  When the chlorine is added to the water supply it will remain in most instances untill the water is filtered with carbon to remove the Chlorine.  The best and safest time to eliminate Chlorine is just before use.  GAC filters remove the taste and odor of Chlorine giving the user great tasting water without any odor.