There are a few reasons why you need to get your chimney swept annually:
Preventing chimney fires - These can be very spectacular with visible flames coming out the chimney pot and a loud noise, or they can be a slow burner and not visible. But they both produce high temperatures and dangers to your home and who ever is inside. These can spread into the roof space and spread through your home and endanger life.
Protecting your health - Having your chimney swept regularly will prevent soot from accumulating to the inside of your chimney which reduces the flow in which fumes can escape, which, in turn can damage your health, and even your life by carbon monoxide poisoning
Preventing smoke damage - By not having your chimney swept regularly, soot accumulates inside the chimney/flue liner reducing the ability for the fumes and smoke to rise upwards. causing potential harmful fumes and smoke to enter the room and leave black staining and soot around the outside of your fireplace and walls and even staining soft furnishings
Prolonging your flue liner - Regular sweeping of your liner will help prolong its lifespan. Depending on the grade of liner, most have a guarantee of around 10 years. But regular sweeping and correct burning will help extend it's lifespan, saving you money in the long term. As it can be expensive to replace a liner