Perhaps someone can come up with a logical reason to do it?
OK, stretching things a little.
If I had an application that would only work at a lower resolution setting, I could run that application on the "analog" screen, set up at a lower resolution. Then I could use the monitor controls to switch between that input and the normal, higher-resolution DVI input.
I've actually seen such technical cases - they were always run with two monitors, to avoid having to toggle the inputs on one monitor. But if you could only afford one monitor......