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

Why is the air filter installed in the air return vents in some houses return air box with filter slot Return air filter box and grille 2 qty MERV 13 18x20x2 inch filters (5 Slot Diffusers & Returns · Spiral Duct Diffusers · T-Bar Plastic Supply return of the dead slot I have attached some pictures that I took of the current condition of the inside of the box by sticking the phone into the air filter slot

return of the dead slot Can we see what the remainder of the return duct looks like that hooks up to that base box? air filter is possible in this case Left

return air support box with filter slot -deep filter The topside of the box is covered with sheet metal with a duct near the middle What this means is that air being pulled into the -deep filter The topside of the box is covered with sheet metal with a duct near the middle What this means is that air being pulled into the