The tray has double side wall for maximum rigidity. If this is not done, the side walls would clump together when heavy objects are placed inside. Reinforcing with a double side allows the tray to retain its shape and give full support to whatever product that you place inside it.
The uses of this tray is not limited the above mentioned tasks, they are also used in bakeries and auto workshops. In regards to the bakery, cookies and muffins can be showcased and displayed in these boxes. As far as the auto workshop is concerned, mechanical tools can be stored in them easily for later access. Furthermore, T-shirts and other clothing products can be placed inside these boxes in order save them from the damage caused by environmental factors. So combined with a nice lid, they can serve for a box as well.