Here is a tutorial showing how you can mix Threejs and Polygonjs scenes together.
This can be useful if you want to use Polygonjs for just some parts of your Threejs scenes for instance. You could use it for parts that are particularly tricky to code like the shaders or particles.
And it can also be useful if you start your scene with Ppolygonjs but would like to integrate Threejs code without having to create a plugin.