The ge_brush is mostly used to control weapon spawns or dropped weapons, though it can be set to any collision group and thus may have a number of creative uses beyond that. For typical use, however, it is either used to keep spawned weapons in place or delete weapons dropped into pits.
Any inaccessible area of a map that a token could potentially fall into should be clipped with this entity. This prevents such frustration as the Golden Gun sliding under a fence or falling down a hole never to be seen again. To block only weapons from passing through this entity, set the collision group to BLOCKWEAPONS.
All death pits in a map should use be covered with one of these entities, using the BLOCKWEAPONS collision group and the Remove Touching flag. For any oddly shaped remove brushes, be sure to check the Use VPhysics for Remove Checks flag as this will make the ge_brush use its collision geometry and not its bounding box for the remove checks.