The thing is that the aspect ratio on most monitors (at least on real computers) is wider than it is tall, and most text pages at least are taller than they are wide. By turning the page so that both tall dimensions are aligned, you can enlarge the view a fair bit more and still have all of what you want to "view" on the monitor.

Most flat panel monitors don't really care which side up they are, so if you happen to have a "swivel mount" you can turn the monitor 90 clockwise and then turn the "picture" 90 degrees counterclockwise (or versey vicey). Some people use them that way all the time.

(I don't happen to have a flex mount on my monitor, so I have to hang it on a rope to do this little trick.)

Some people do think it's important to be able to make their computer look any way they want it to look, and to be able to do what they want it to do. Obviously, Microsoft doesn't agree with them.

John