Air cleaners and purifiers filter certain allergens and particles from the room they are in, with a HEPA filter being the most efficient. They are designed to improve the air quality by filtering dust, smoke, animal dander, and other minute particles from the air. They do not eliminate dusting of surfaces.
If your room has an excessive amount of dust, smoke or animal dander, the air cleaner's efficiency may greatly be reduced, and filters will require more frequent changing.
By dusting regularly, you will help the air cleaner more effectively filter the minute airborne particles that cannot be removed by dusting alone, and this will reduce allergens in this room.
How often should you dust? At least once a week or more often if you can see any accumulation of dust on surfaces, and remember to also vacuum dust the air cleaner's exterior grills and panels.