Because.
It’s basically creating actually black areas in the image.
Think about it this way.
Take a render cloud, make the cloud slightly darker.
If you do this in photoshop the end result is already in 0 to 1 range. Not the same in engine.
So, if you were to ADD that without the clamp to something, you’ll be effectively making that something -2 in some areas - which is unpilely to be the needed result.
Particularly when you then multiply it by something else.
Whereas X by 0 is?