Every chimney needs a cap for the same reason every house needs a roof. A chimney cap can prevent rain from entering the chimney causing the damper and metal firebox to rust as well as causing damage to the flue liner.
In addition to prevent water entering a chimney cap prevents raccoons, chimney swifts, bats, squirrels, and other animals from entering the chimney. Squirrels, raccoons and birds are drawn to chimneys because they provide a safe place for their young. These animals not only can bring mites, ticks, fleas, and other parasites, but can cause a blockage in your chimney which would not allow the products of combustion to vent to the outside like they should.
A stainless steel chimney cap with wire mesh that is small enough to keep animals out but large enough to allow the products of combustion to escape. Having a chimney cap installed on your chimney is by far the most economical way to prevent water and animals from entering your chimney.