There are different kinds of primitives. The most basic primitives are those modelling shapes,
such as Sphere, Triangle and the plane primitives.
A more abstract kind of primitives model operations on primitives: Decorators
assign materials to other primitives while Transformers can be used
to move primitives around in space. The BoundingBoxAccelerator primitive
is even more abstract: it changes absolutely nothing about the scene.
If it does, there’s a bug.
The BoundingBoxAccelerator's only purpose in life is to pass the butter
speed up rendering. Say you have a scene containing a complex object built out of
thousands or millions of triangles (i.e. a mesh):
Each time a ray is cast (e.g. from the camera, for shadowing purposes, reflection or refraction), we need to check whether this ray intersects any of these triangles. Rendering would take an inexcusably long time if we were to do nothing about it.
The solution is to surround the mesh by a box. This box needs to be as small as possible, but still contain the entire mesh. When we need to determine whether a ray intersects the mesh, we first check if it intersects the box. If the ray misses the box, we know it will also miss the mesh, thereby sparing us the need to iterate over all triangles in search of hits.
By building a hierarchy, you can even speed up rendering more.