It's common to take map geometry from a level editor and process the polygon soup into some kind of space partitioning tree structure, but here the octree is manipulated more directly. The idea is to accept some limitations on what shapes can be created, for the sake of better real-time editing performance.

The original Cube engine is almost as old as Quake III: Team Arena, which can do more free-form geometry and has pretty light-maps, but ‘compiling’ these maps was a lengthy, compute-intensive task at the time, which made online multiplayer in-game level editing impossible.