Chimney caps are designed for a variety of reasons. Chimney caps keep water from entering the flue during heavy rain storms. They help keep animals from entering the chimney flue and making a home. During high winds, the wind can sometimes blow down the chimney. Chimney caps can prevent wind-induced downdrafts. Another good reason to have a chimney cap is that these caps have screening along the sides, which are called spark arrestors. This screen is designed is designed to prevent hot embers from rising up the flue and landing on the roof. Hot embers on the roof can catch the roof on fire.
There are a few colored chimney caps. Black caps are galvanized and rust very easily. Silver caps are stainless steel and do not rust. Finally, there are copper chimney caps, which are very expensive and are usually a special order cap. These are typically ordered as a cosmetic function for the home.