Dirty chimneys can cause chimney fires which can cause severe damage. Fireplaces and wood stoves are designed to contain fires safely. The chimneys job is to expel the byproducts (water vapor, smoke, gases, etc.) that fire produces. The residue that these byproducts create that sticks to the inner walls of the chimney is called creosote. If creosote builds up significantly, and the internal flue temperature is high enough, the result could be a chimney fire.