The blower has to work harder to move air through the filter, and if a filter becomes clogged or if a replacement has been delayed, the amount of air circulating can be significantly reduced. If all you need to do is replace a dirty air filter or a washing machine, you can probably do it yourself. As dirt and particles accumulate on the filter, they can obstruct airflow, so fitting a new filter means you'll enjoy more powerful air in your car's cabin. Most of the time, the filter fits in a slot in the HVAC box that has a small door that covers it and is usually easy to replace. The purpose of the filter is to clean particles from the air and prevent debris from accumulating in the HVAC box, whether it is fresh air from outside or recirculated air from inside the vehicle.
Furnace filters generally need to be changed more frequently during periods when the HVAC system is most used. If you have a cheap air purifier, it might be better to just replace it if it has problems. In fact, this is a repair that you can do on your own, which means you'll only have to pay for the filter. If you feel that not much air is coming in through the vents or that unpleasant odors are leaking out, it's probably time to replace the filter. In most cases, you'll only have to pay for the cost of a new filter and there shouldn't be any other parts that need to be replaced.
Also, if you have an inexpensive air purifier, it probably makes more sense to replace it rather than calling a professional. Some of the more expensive ones or those in high-end cars can be activated with charcoal, which helps filter fumes and toxins from the air. The frequency of filter change will depend on the filter manufacturer's recommendations, its efficiency, how often you use your car's heating and cooling system, and the characteristics of your household. You should check your filter packaging to see how often the manufacturer recommends replacing it with a new one. The cabin air filter prevents dust, pollen and other debris from entering the cabin through the climate control (HVAC) system. Replacing an AC filter doesn't have to be expensive or difficult.
In most cases, all you need to do is purchase a new filter and install it yourself. This is an easy repair that can be done without calling a professional. However, if you have an expensive air purifier or one in a high-end car, you may need to call a professional for help.