Chemical filter media removes pollutants from your water that biological and mechanical filter media cannot. These pollutants come from fish medications, organic compounds produced by various biological processes, and pollutants in the air. If you have an undesirable odor or color in your aquarium water, it's likely that chemical filter media will remove it. Always keep a close eye on the water conditions and trace elements in your tank.
Activated carbon is coal or coconut based; other chemical media - ion exchangers - are made of zeolite or synthetic resins. Each has its strengths and drawbacks, so check both sections to see what's right for your aquarium.