This is one of the interior walls in a Bee Tree Hive. You can see that they have smoothed it out by propolising the rough sections with a beautiful, red propolis coating.
This is what they do in natural hive cavities in the wild. The benefit to the colony is that they are living in a propolis envelope with all of its natural anti-viral and anti-bacterial qualities. This is far healthier for the bees than smooth, un-propolised Langstroth hive bodies.