Anyone who has used a clothes dryer knows that a significant amount of lint is generated and that the lint trap needs to be cleaned. Depending on the fabric being dried, the lint trap generally needs to be cleaned after every load of clothes. Cleaning the lint trap improves air flow and lets it operate more efficiently.
The lint trap does not catch all the lint. Some lint may make it all the way through the dryer air vent and be released outside of your home but some of the lint can get stuck in the duct work. Flexible dryer hose that has ridges and corners and bends in duct work are especially vulnerable to accumulate of lint. The longer the duct work the greater chance that lint will not make it all the way to the outside of your house as the air from the dryer that pushes it along weakens the further it travels.
A surprising number of home fires are caused by dryer vents that become obstructed by lint. U.S. Consumer Product Safety Commission (CPSC) studies have shown that in excess of 15,000 home fires are associated with clothes dryers and excess lint build up is suspected in many of these fires.
Lint is highly flammable and if a dryer vent becomes restricted and cannot properly vent the hot air, it can cause heat to build up which can ignite the lint. These fires are easily preventable with a little preventative dryer vent cleaning.