I'm aware of that, but sometimes I need objects in an animation to be highlighted for example. It's easy to render out a mask and raise levels in post, until it goes behind glass or something. For things like that you want the full material + self illumination but without emitting actual lights.
Lots of times I now use the LightMtl to render objects with baked textures for example, put the texture in the light material and disable emission and you have a nice way to render texture as they are without being affected by any lights. But let's say you do want some reflections on it you can't do that easily. You have to do the same trick with a normal material which will always emit actual light. .
So that's why I asked for that checkbox.