OnWillRenderObject is called once for each camera if the object is visible.
This function is called, if the object is deemed visible from the current camera after the culling process.
The method is useful, if you need a preparation step for every camera that is rendering the object.
One example of this is rendering reflection onto a render texture.
The reflection will be different for every camera's view point, and needs to be rendered before the original object.
If the object gets culled away by a given camera, the reflections for that object will be skipped.
For usage in a proper context, see the script Water.cs in Assets->Import Package->Effects.
Note that Camera.current will be set to the camera that will render the object.