It is common for birds to enter a chimney that does not have a chimney cap, typically looking for a nesting area. The birds can also fly into the fireplace and, if the fireplace door is open, right into the house.
Animals also are known to get into chimneys, particularly squirrels, raccoons and bats.
If nesting material in the chimney is not removed, the blockage can cause carbon monoxide to build up in the house when the fireplace or wood stove is used.
Rain coming into the chimney can cause unpleasant odors and damage flue walls. Rain also may run down into the stovepipe and the stove itself, or into the fireplace, and cause further damage.
Another purpose of a chimney cap is to minimize wind downdrafts that cause smoke.
A chimney cap prevents embers and sparks from flying out of the chimney and possibly damaging the roof.