The fire risk
Why Lint in the Vent Is a Fire Risk
Lint that slips past the screen settles inside the vent run where the dryer cannot reach it, and every layer chokes the airflow a little more.
Fire-safety bodies such as the National Fire Protection Association (NFPA) cite clogged dryer vents as a cause of house fires, and the United States Fire Administration publishes dryer vent fire-safety guidance.
A dryer pushing hot air into a choked run works harder for every load, which costs efficiency long before it becomes a fire hazard.