With visual effects it makes it possible to give a creepy ambience. It's possible to change light style in maps and set dissolve effects on players' bodies when they die.
-Its supposed to have support for adding fog, but because of technical limits in SourceMod it's not +
It's supposed to have support for adding fog, but because of technical limits in SourceMod it's not fully functional yet. For developers who wonder why, it's because there must be a env_fog_controller entity in the map. That entity isn't networked so SourceMod can't create that entity type (Logical Entity Support is a work