Prior to AutoCAD LT 2010, you couldn't rotate paperspace viewports. (If you tried, it just sort of moved around the screen, maintaining its original orientation.) If you wanted to show a viewport at a different orientation, you had to go through a multi-step process involving the UCS and PLAN commands...big hassle.
Now, though, you can change a view's orientation simply by rotating its viewport.
Here's the original view:
And after using the rotate command on the viewport:
The view automatically updates to match the new orientation -- no need to mess around with the UCS or anything.
This is the default behavior, but if you don't like it, we've given you the option to turn it off. Setting the system variable VPROTATEASSOC to 0 means that only the viewport boundary rotates, not the view inside it.