I think they do some kind of limited VIS space... For example, you know that world brushes tend to split other world brushes in triangles when they?re connected. If a simple (let?s say 6 faced) brush is connected to a floor brush with 4 sides (imagine a pillar over the floor) the floor will be splitted in 8 triangles (try to draw it in a top view and then draw lines to connect all vertices so they all make triangle shapes... I can?t explan myself nicely, I?d draw it for you if I knew how to post and host pictures).
Imagine now that the pilar is 8 faced... The floor brush will be splitted in much triangles so maybe, to increase Rspeeds you want to put a hint texture surrounding the pillar so it?s run with a separated VIS... Well, I think it?s something like that but don?t trust me.
These HINT textures are included in ZHLT.wad.