Read the quake pvs article linked to in this thread. On top of my head as I remember it.. one of the reasons for Quake's use of a bsp was to allow back to fron

Read the quake pvs article linked to in this thread. On top of my head as I remember it.. one of the reasons for Quake's use of a bsp was to allow back to front rendering of the world geometry without the use of a zbuffer. This was required to get decent performance with the software rasterizer. I'm not 100% sure what's most commonly used these days, but for a large open world requiring data streaming, I could see the use for something like an octree and even portals.