I sometimes, for debugging purposes, need to type some long, obscure hex string in one editor from another machine (don't ask). Using a keystroke to switch windows causes me to lose my place. A two-screen solution would be helpful in this case.

Also, sometimes I like to play music videos on youtube while doing other stuff and it would be nice not to have to context switch away from the concert. Really, there are quite a few handy applications for a separate physical display.

I use my phone in a cradle as a second screen but it's not ideal.