Maybe you do know three.js and need to use it. React Native modules are how your code can directly include raw three.js programming. This is very useful if you need to programmatically create native three.js objects, modify material attributes, or use other three.js code that isn't directly exposed by React VR.
You may have some JavaScript code that does business logic, and don't want to, or can't rewrite that as React VR components. You might need to access three.js or WebVR components from React VR. You might need to build a high-performance database query with multiple threads so that the main rendering loop doesn't slow down. All of these things are possible with React Native.
This is a fairly advanced topic and will not normally be required to write engaging, effective WebVR demos; still, it's fantastic knowing that React VR and React are so extensible.