Look at it this way, your furnace outlet vents supply the warm air to the unit, it circulates inside where we live and move around, where the cats, dogs, Ferrets and assorted carbon-based units move around. The air then goes under the steps (in my unit) into the basement area and back to the return air part of the furnace. The basement area is just a space that air goes thru to the return furnace, like a big duct.
The filter.. filters all the dust you kick up, all the Dog, cat and Ferret hair, dander and dirt that follows you in and circulates around the space the door when it opens.
I want as much of that air filtered as possible when it exits the floor registers into my sensitive nose and down my windpipe to my pristine lungs
If a fella does not care about that, then don't filter the air.
