Imagine you have both a desktop and a laptop, with your laptop screen positioned below the display connected to the desktop. You can make your Logitech mouse act as if they were one device; if you slide past the top edge of your laptop screen, both the mouse and the keyboard switch over to the desktop.

You can even press ctrl+c on one device, move your mouse to the other and press ctrl+v (with the same keyboard of course), and it's going to do the right thing. I think even drag and drop across computers works, at least in some circumstances.