The NULL texture is primarily used to reduce world polygon (wpoly) counts by applying it to a face the player is normally unable to see in-game. When applied to a face - it tells the compiler to remove it, thus reducing the amount of rendered faces in certain areas of your map and improving performance.
The texture blocks VIS and is solid, so it can be applied to brushes facing the void.
See also:
r_speeds,
HINT,
BEVEL