Cleaning out your lint trap and lint in the vent line is important for more than just making your dryer run more efficiently. Dryer lint left unchecked can translate to a build up of lint inside the machine. While you may not realize this, that lint is extremely flammable. It's essentially very airy cloth remnants and doesn't take much to ignite. Just a spark or two from your machine can start a fire that can quickly get out of hand. Not all lint gets caught up in the machine. Some of it continues out of your dryer and into the duct leading from the dryer to the outside of your home. There are numerous points along the path where it can get stuck, on the walls, around a curve or even at the termination point in the vent. This is another area you need to have regularly cleaned by a professional to keep build up to a minimum.