"Why use DIF?" is a common question among beginning developers. It seem like it would be much more convenient to create buildings in a polygon editor like 3D Studio Max or Maya, and indeed it would be. There are some very real technical reasons that we use a BSP Brush editor like QuArK.

The BSP Brush style of object creation, while somewhat limiting in the creation phase, allows for an incredible number of optimizations on the code side of the engine. BSP Brushes are a very efficient way of determining object collision - one of the most CPU-intensive processes a real-time application performs. It also provides numerous shortcuts for culling of visibility through the use of "portals" so that rooms and terrain that the player can't see don't get sent to the graphics card for rendering. This is a lot harder to do, from a mathematical standpoint, than a non-programmer might imagine.

For a helpful simplified overview of BSP Brushes and how they work, check out this document:

BSP Technical Details For Dummies