This prevents the deeper nojibg box from buckling when you stack shallower boxes on top for 2-level organizing. They are super cheap, but the plastic is too flexible. The tie bar fixes that shortcoming. Just clip it onto the edges of the big box.

There are a couple of tabs on either end which you need to cut off after printing - the are meant to keep the the first few layers of the clips in place so that you may print it without further support material.