Leaks

If you have any Quake or Half-Life mapping experience, you will be familiar with the concept of leaks in your level. Leaks are caused when your level geometry (brushes only) does not entirely seal your level. By definition, a BSP must be a completely sealed interior environment; even if you are planning to show a massive outdoor terrain through a skybox, the underlying geometry (the BSP) will need to be sealed.

This is why using Snap to Grid is a crucial time-saver: It ensures that your brushes are properly placed so that your level doesn’t have leaks.