This article is from the Water for coffee FAQ, by Jim Schulman with numerous contributions by others.
Charcoal removes chlorines, odors, colors, and cloudiness, but
does not affect the water's mineral content. It is an excellent idea
for poor quality water, when chlorinated water is drunk directly
without heating it or letting it stand, or when chloramine instead of
free chlorine is used for disinfection. Charcoal filters are fairly
low maintenance, needing only occasional cartridge swaps. Brita, Pur
and Culligen cartridges always include charcoal filters. Most
installed treatment systems also have one, as well as one or more of
the following items.