How to make a cube out of six bounding boxes

I’m trying to re-engineer a tutorial I saw on how to Generating Procedural Planets with Level-of-Detail: 3D World Generation: #4 (Planetary LOD): Generating Procedural Planets with Level-of-Detail! - YouTube

you can jump to the 1:02 mark he starts to dive into the code. I’m not sure why he opted to use a Box3 to define each side of the box but I want to find out the reason.

The code can be found here on line 87: ProceduralTerrain_Part4/quadtree.js at 97ffc7ecb1624b274416e071fd8926165ca59d7a · simondevyoutube/ProceduralTerrain_Part4 · GitHub