I think tones become brighter and more percussive with harder picking. Therefore if you happened to need to make your amp sound brighter, during a softly picked or finger picked section, and you wanted to remain in that same tone when you started using your pick, but that brighter tone that was great while picking softly or finger-picking becomes shrill, you can offset and "darken" it with this setting. That is an extreme "what if" scenario, but I'm just sayin' it could have it's uses.
The next thing that comes to mind, is maybe copying the amp into x and y, but having one of them darker by use of this parameter, because you are going to use a different pickup at a certain part of the song, but you want the same gain structure, you just want to take a little harshness off of how that other pickup sounds, compared to your primary pickup. Perhaps, you are also boosting the scene's output volume for that passage, and at that slightly higher output volume, it just bites too much, and you want to shave that off a little.