Every time you use your dryer, the wall dryer vent experiences a build up of lint. The lint trap in your dryer catches much of the lint, and that is why it is important to keep the lint trap in your dryer clean.
But lint will escape the trap and build up in the back of your dryer and in your wall dryer vent.
Enough lint builds up that you should clean the back of your dryer once a year and clean your wall dryer vent at least once every two years. Cleaning out the lint will help the dryer to run better and reduce the chance of a fire.