the filter in your blowdryer catches dust and debris to keep the motor from overheating, which extends the life of your blowdryer. the idea is similar to cleaning the lint trap in your dryer after every load of laundry.
lucky for you cleaning your blowdryer is super easy, it just gets overlooked by most. i clean my blowdryer in the salon once a week, but at home i'd say about once a month is enough. i use a paul mitchell express ion dry in the salon and at home, if you're looking for a new blowdryer i couldn't recommend this one enough! dries quickly, lightweight, quieter than other blow dryers and if you treat it right it'll last you a long time. (mines limited edition, so they no longer have the same wooden pattern! sad i know)
i realize some may have never cleaned their blowdryers before, and i'm hoping you notice a big difference when you do! remember to do this regularly to keep your motor from overheating (which can also be a fire hazard), because once it does you'll have to buy a whole new blowdryer!