The plastic housing is designed to channel or direct the airflow onto the GPU cooler and through the fins. With regards to mounting  fans front or back it depends how you mount the (direction). you should not mount them blowing against each other. Mount on the back blowing forward and on the front sucking forward. That is what I did because of a big box around my rig. But if you have an open rig it is best to just mount some fans behind blowing air through your stack of GPUs.