I could be wrong, but I think it is so you can hook up more then one computer to your monitor, or another device that uses DVI.
I don't believe the monitor is restricted by just using one DVI using resolution 1920x1200, it isn't till you hit the 30"s displays that you need DualLink DVI and where you go above the resolution 1920x1200.
Please correct me if I am wrong, it has happend before :-)
Henrik