Bad chimney crowns are one of the #1 sources of chimney leaks. Not only does it allow moisture into the exterior brickwork and interior liner, eventually it will make its way to the fireplace, deteriorating the firebox, interior mortar joints, smoke shelf, and causing the damper to rust out.
A chimney cap is essential for keeping moisture out of the chimney. Next time it rains, put a bucket outside and see how much water it collects. That amount of water is collected in the interior of your chimney every rain. This moisture causes all kinds of deterioration and also be a breeding ground for Mold & Algae.
Spalling brick allows moisture to get into the exterior brick of the chimney. If left unattended it will continue to destroy the whole chase of the through deterioration and freeze & thaw in colder climates. It can also allow moisture into ceilings, attic spaces, interior drywall, causing expensive repairs, and even mold issues.
Common areas of leaks; Chimney hips are where the chimney corbels and often hold water. It is a breeding ground for algae and does not take long to deteriorate mortar joints and the masonry around it. If you are experiencing a chimney leak or an excessive amount of moisture in the firebox area, chances are the hip of the chimney is leaking.
The flashing is designed to protect the area where your chimney protrudes through the roof, It plays an important part in keeping the interior dry. Unfortunately so many times it's neglected and when it becomes an issue, many contractors and handymen put a temporary fix on it i.e. caulk, or roofing tar.
The prefabricated chimney type is a metal flue pipe system designed to vent smoke/ gases out of the house. Prefab units function similarly to a brick-and-mortar masonry system but are designed differently. Instead of a masonry crown, these units have a metal chase cover usually constructed of galvanized metal. These chase covers often hold water and rust through after a period of years. When it's time to replace it, I strongly recommend choosing stainless steel to avoid it rusting through again.