It is needed in a specialization of `LeafContainerTraits` for `OctreeCentroidContainer` ([here](https://github.com/jpapon/pcl/blob/octree_container_rework/octree/include/pcl/octree/octree_pointcloud_voxelcentroid.h#L136)). In the rule that enables/disables that specialization we need to have access to the point type template parameter, which is impossible unless it is explicitly typedefed within the class.