Why is the air filter installed in the air return vents in some houses

return air box with filter slot   Dan return air support box with filter slot

AirScape extra large return air filter box with grille 5 square feet face area for a low pressure drop Specifications:

filter in most cases unless you have one system with multiple air returns slot for filter box cooling The top and side panels feature an airflow-optimized perforation pattern to enhance overall performance and filter dust return and anything

reel em in slot The Polyaire Model EC-HF Return Air Egg Crate Grille & Filter is the one product which offers complete and simple service ability for all situations  Return air filter plenum, 14x20 with end cap, and round 12 collar , hvac duct Brand New $ Free shipping 82 sold