The parameters defined in the [WorldEnvironment] can be overridden by an [Environment] node set on the current [Camera3D]. Additionally, only one [WorldEnvironment] may be instanced in a given scene at a time.
The [WorldEnvironment] allows the user to specify default lighting parameters (e.g. ambient lighting), various post-processing effects (e.g. SSAO, DOF, Tonemapping), and how to draw the background (e.g. solid color, skybox). Usually, these are added in order to improve the realism/color balance of the scene.