Originally posted by NeoStrider
at the same time when you think about it, if a light doesn't illuminate then how can it cast shadows?
a shadow by definition is an area lacking in illumination from a light because another object is blocking the light rays.
yes, but creating lights in maya doesn't necessarilly create the shadow that you want - using lights seems to be a process of cheating to create the effect that you want. So while a shadow might be theoretically correct, you might want to cheat and make the shadow longer, to make the shot more aesthetically pleasing. In that case you would need another light to create the shadow, but you would not want that light to create more illumination.
Another example might be deforming your character's arm to increase the effect of foreshortening to the camera. However, his shadow would look wierd, so you would need to create another non illuminating light and possibly an invisible object to create a 'correct' shadow.
Obviously you are using these methods with other lights in your scene, so it's not as if you would be creating a shadow without a source of light - you are just cheating to create an illusion.
Although I probably haven't done a good job of explaining, these are both examples taken from 'Digital Lighting and Rendering which I definately recommend if you haven't looked at it.