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

Return Vent Filters return air box with filter slot Take advantage of the durable 30-gauge zinc-coated, galvanized steel construction of this Master Flow 16 in x 25 in Return Air Box to equip your HVAC return air box with filter slot The 16x25x1 filter normally drops into a slot in the short horizontal run between the air intake downward duct and the furnace itself There

return air support box with filter slot air out, but ALWAYS pulling air back TOWARDS it Your filter slot is almost always on this