Chimneys are constantly bombarded by the elements. Resting above the roof-line virtually defenseless, chimneys do not have any roof or gutter system to divert water away from them unlike the main part of a brick building.
A simple hairline crack in your chimney capstone, if neglected, can cause serious damage. Water penetration through a failed cap can cause the top courses of your chimney to disintegrate to the point where masonry repair is mandatory. When brick on a chimney is spalling, flaking, cracking, displacing or even missing, it may be time to rebuild it with new masonry units.