The lint trap catches a lot, but not all of it. The rest settles inside the vent duct, and over months it narrows the path the hot, damp air needs to escape. Here are the signs it's time for a cleaning.
One: clothes need two cycles to dry. Two: the outside of the dryer or the laundry room gets unusually hot. Three: you notice a burning or musty smell. Four: the exterior vent flap barely opens when the dryer is running. Five: it's been more than a year since the duct was cleaned.
A restricted vent makes the dryer run longer and hotter, which wastes energy and wears out the heating element and the thermostat. More importantly, lint is highly flammable, and dryer fires are a leading cause of home fires involving appliances.
A professional cleaning clears the whole duct from the dryer to the outside wall with rotary brushes and vacuum, then checks the flap and transition hose. Most homes should have it done once a year, and more often with a large family or a long duct run.