The light_environment entity is a light controller, that casts both diffuse and parallel directional lighting from the toolsskybox texture. This is to approximate the light cast from the Sun or Moon.
The properties of this entity apply to the whole map. Multiple instances of this entity within a map may cause errors. A 3D Skybox does not require an extra light_environment placement.
Light_environment is meant when u have a skybox.If u dont then it wont work in the map.
U can find information about light_environment here :