The main reason why furnaces and cars have it on the incoming side is because the dust and particulate can damage the engine/blower. So you want to filter it ou

The main reason why furnaces and cars have it on the incoming side is because the dust and particulate can damage the engine/blower. So you want to filter it out before it reaches there. In a system like this, the box fan will not be damaged by the particulate so you can instead push the air through the filter instead of pull it.