The small cavity is probably mostly hidden behind the food in your case too. She is responsible for regulating the amount of condensation in the refrigerator.
The condensed water that collects on the back wall runs off through the hole via an inclined device. From here it flows into a container from where the water evaporates. If the water cannot drain through the hole, small puddles can form in the refrigerator. Therefore, you should regularly check whether the hole is clogged or needs cleaning.