This feature is probably based on the old word processors that used a monitor that more closely matched the page you were writing on. Now days with the flat monitors you are basically 4 screws from turning it 90 degrees. The mounting bracket has 4 holes in a square pattern. If my machine was rendering 8.5x11 documents all day long I would flip it.
Probably a handy thing for a FEW people. MS decided we all needed it.