See the NBC News segment on dryer fires.

This is 2 gallons of lint
taken from one apartment.
Imagine the fire hazard
for the entire complex.
Dryer vents should be cleaned once a year.
Depending on the dryer, it’s frequency of use,
the length of your vents,
your vents may need attention more often.
Save money on your electric bills.
When your dryer vents are blocked, you will need more cycles to actually dry your clothes.
Save money on appliance repair and replacement.
Not only does your dryer get more use when your vents are blocked, it also has to work harder and it wears out faster.
Reduce fire hazards in your home
Lint is flammable. Excessive lint makes it more likely lint will blow back into the appliance and ignite. Every day there are more than 40 dryer fires in the U.S. alone.