What is in unity_WorldTransformParams.x, y, z?

Are they useless now? Can I use them to store some custom values?

 

Yes.

No.

The variable was added some time during first few versions of Unity 5, and AFAIK the first time it showed up it only used the w component. Why? I'm guessing only the person who added the variable knows, and may not even remember anymore. And you can't override it with a custom value as Unity sets it just before rendering, so anything you try to use will get blown away with (0,0,0,1) or (0,0,0,-1).