I'm guessing the same way you'd use them in GL.
RELAX! I wasn't really going to leave you with that.
I'm not really up to speed on Java3D, but I'm assuming that these are collections of points that descrbe a set of lines, or triangles. For example, if I was drawing a cube, I could start with one vertex and keep drawing lines until I completed the cube. This way I'd only describe each vertex once instead of 6 QUAD polygons or 12 triangle polygons. Of course, why you'd use these directly in a scenegraph that handles most of it for you, is beyond me. They're probably there for the SceneGraph's use and where made public in case you wanted to create your own object types.
Does that help?