You will find this situation, where you just need a shadeless, common and very useful throughout your work, mainly because Cycles offers rendering with GPU, and it’s easier to set this up just for the object, than to go back to Blender Render and set the whole scene for this.
When you bake the rendered image for example, this setup prevents your character from being affected by it as if it were a light source. When you do a screen, for example in a tracked cell phone, that you don’t want/need to affect the environment (therefore, increasing render time) you’ll need this. In general, when you only want the image as is, this is the setup you look for.