What a missing cap costs later
Rain that falls into an open flue runs down the flue wall, soaks the smoke chamber and the firebox, and works into mortar joints from a side nobody can inspect. It lands on creosote, which is where the smell after rain comes from. It freezes in the joints through the winter and opens them. Fitting a cap is a small job, and the repairs it prevents are not.
Chase covers and the rust stain
A framed chimney, the kind clad in siding or stucco instead of built from brick, is closed at the top with a metal chase cover. It has to be sloped, cross broken and edged so that water runs off and drips clear. A flat galvanized lid holds a puddle after every storm. It rusts through from the top, the water goes into the chase where the framing is, and the first sign at ground level is a rust colored stain running down the outside of the chimney.
The damper, and the two places it lives
A throat damper is the plate immediately above the firebox, operated by a lever or a handle inside. It is cast iron on a frame set into the masonry and it fails by rusting or seizing, and once it stops seating, the flue is effectively open all year. A top mounted damper is fitted at the top of the flue instead, sealing against a rubber gasket and pulled shut by a cable running down to the firebox. The second one also keeps rain out when it is closed, which the throat plate never did.
The animals
An open flue is a sheltered vertical shaft and it gets used. Birds and squirrels build in it, and a nest in a flue is a blockage. If that stack also vents a furnace or a water heater, a blockage stops being an inconvenience and becomes a carbon monoxide path. The screening in a cap is what stops it, and a cap fitted after an animal has already been in there is fitted after the flue has been cleared, never over the top of a nest.
Why a stack sometimes needs more than one
A chimney often carries two or three flues: the fireplace, the furnace and the water heater, each with its own tile. Capping one and leaving the others open closes a third of the problem. A multi flue cap covers the whole top of the stack in one piece and is measured across the crown instead of to any single flue, and where the flues are at different heights that measurement is the whole job.
When it gets noticed
Cap and damper faults surface at the start of the heating season, when a fireplace that worked last year will not draw, or the room smells, or the first fire fills the house with smoke. That is also the busiest part of the chimney calendar, from late August through November, so a cap noticed in September is a longer wait than the same cap noticed in June.