If, as JASP explains below everyone is referring to rotary as twisting or turning of the legs to produce rotation of the skis in the plane of the skis then I was wrong in my statement about migration of this discussion. I hope you realize that my confusion is honest. Both in my discussions in this thread and from many other posts I have heard that rotary refers to joint and body movements. Thus, when I hear it used I think about it in those terms and maybe wrongfully assume that it only results in a twisting or pivoting action of the skis part of the time.
That's why I revisited the question about the difference. You see, there are effective rotary movements and ineffective rotary movements. Still, even ineffective rotary movements can be useful at times (e.g. turning the head and shoulders while airborne). It was assumed that when you asked about why to focus on twisting, that we were talking about effective rotary movements that result in skis pivoting under the feet as opposed to pivoting around the tips.
As to the original question on when to focus on twisting/rotary, you can choose to focus on it in order to buiild upon a strength or to work on a weakness. You can do this without weakening the argument that twisting works best when blended with tipping and pressure. There are times when one wants to blend and times when one wants to isolate.