Isn`t the "PrimitiveOctree" of "FScene" used for speeding frustum culling

The “FScene” class has a “PrimitiveOctree” (a object of “FScenePrimitiveOctree”) containing the primitives in the scene.
I guess that this “PrimitiveOctree” is used for organizing and managing the objects (except the lights, there is another “LightOctree”) in the scene. But, I dont find where using this "PrimitiveOctree" in UE4 engine, such as "Frustum Culling". Or isnt the “PrimitiveOctree” used for speeding frustum culling? Then what`s the “PrimitiveOctree” used for? How to speed frustum culling in in UE4 engine?