A rain cap is not required for a chimney, but it is recommended. A rain cap keeps water, debris and animals from entering the chimney.
Without a rain cap, water and snow can enter the chimney and cause damage to the interior of the chimney. Small animals, such as squirrels or birds can build nests or enter the home. Debris also piles up and can cause a chimney fire if not removed before building a fire in the fireplace.
Rain caps also prevent sparks from leaving the chimney, where they can set surrounding roofs or ground debris on fire. They also prevent downdrafts from entering the chimney and blowing smoke into the room.