Skyline_UK
Not sure why you'd need to open multiple PRVs. I open as many tracks as I need in one PRV and click on the relevant button on the right to view whichever track(s) I want to work on. Or am I missing something?

 
And another reason is to quickly display only a specific group of instruments in the view and efficiently edit them without affecting, or being distracted by, other groups of instruments. You can, for example, display only violins in one PRV, only violas in another, only percussion instruments in another, and so on. Changing the views requires only one click of a tab.
Lock the content and lock the Screenset, and you'll see how convenient the feature is.