Carbon block filters are often the second stage to a reverse osmosis system. Carbon blocks remove chlorine that can be harmful to your reverse osmosis membrane (the membrane is the most important and most expensive filter).
Catalytic activated carbon is used for chloramines removal from the water supply in *some* cities. If you are not sure if your water has chloramines, please contact your municipality regularly. If you are unsure, you can use a chloramines carbon block for added insurance without worrying.