A cracked cap allows water to penetrate the chimney causing premature deterioration and in northern climates, freeze/thaw damage.
The purpose of a chimney cap is to prevent water from penetrating the top of a masonry chimney. The chimney cap should not be confused with the rain caps which sometimes cover the tops of chimney flues to prevent rain water from running down the inside of the chimney. A chimney cap is usually constructed of concrete; however, some are stone or metal. A good quality cap normally overhangs the sides of the chimney at least one inch to provide some protection for the chimney from water dripping off the cap.
In many cases, a proper cap is not provided. Bricklayers often put a thin coat of mortar over the top of the chimney (exclusive of the flues, of course). With time, this thin layer of mortar cracks and eventually becomes loose. Examine your chimney cap using binoculars. If you see cracks or deterioration have it evaluated and/or repaired.