The National Fire Protection Administration (NFPA) has stated that even 1/8th inch of creosote in your chimney can be a fire hazard to your home. A buildup of creosote can catch fire inside your chimney, resulting in a 2,000 degree full-blown chimney fire. These infernos can wreak havoc on your chimney, often times escaping into the attic, walls, roof, etc.
The first step to preventing a chimney fire is removing the creosote deposits which are a natural byproduct of burning wood. Sweeping away these deposits is the primary role of a chimney sweep.