Rumour has it that chimney cowls derive their name from the cowl garments worn by monks!
Smoke produced from stoves or open fires to heat rooms rises through the chimney and into the stack and outside via the chimney pot. Warm air created by the combustion within the fireplace or wood burning/multi fuel stove will rise up the chimney, under normal conditions, taking any smoke with it and dispersing it high up into the atmosphere where the smoke is less of a nuisance.
If you ever see smoke coming out of a chimney pot that seems to be blowing down the roof towards the road, this means the atmospheric conditions are causing a down-draft. In very strong, cold and/or windy conditions, any breeze may overwhelm the updraft and push the airflow in reverse back down the chimney. As a result, smoke can then fill the room that houses the fireplace or stove. This poses a health risk (CO2 carbon monoxide poisoning) and chances are that soft furnishings in the home then need to be professionally cleaned.
The amount of smoke given off is considerable when real coal is burnt (as it used to be for hundreds of years before the Clean Air Act legislation took effect). So fitting the correct cowl is essential or you will encounter smoke leakage into the nearby room.
Another important function of a cowl is to prevent birds and squirrels from nesting in the chimney. Cowls will also prevent rain, snow and even hailstones coming down your flue. Hailstones mixed with soot will have a detrimental effect to any cream-coloured carpets.
Cowls are also used on chimneys where the flues are redundant and therefore need to be capped off for protection against ingress of precipitation and Jackdaw birds nesting. A number of people also cap-off their chimneys to reduce heat loss since warm air from the central heating is being sucked up the chimney.
If you want to stop using your chimney, please remember that it still needs to breathe. It is wise to use a cowl that is designed to lend protection to the flue but also allow a degree of ventilation.
If you are local (Tunbridge Wells, Tonbridge, Sevenoaks, Brighton and Hove etc) and I am sweeping your chimney, I would be more than happy to give you some advice on which cowl would be best for your chimney set up.