Lint accumulation and reduced exhaust airflow feed on each other to provide conditions ripe for a fire. Lint is highly combustible and decreased airflow causes overheating of the exhaust environment, demanding excessive cycling of the high temperature limit switch and eventual failure.
If your clothes are taking longer to dry or if the velocity of air exhausting from the dryer vent hood is minimal, maintenance is needed.
Over time, your dryer vent fills with lint that sneaks by your dryer's filter, which makes your dryer work harder to dry your clothes. You can save $$ every month on your dryer bills if you keep your vent system free of lint.