Many editors allow you to open files on the side of your screen. It is specially usefull on widescreen monitors. So why not to do that for the internet browsers. It will open new degree in usability.
For example you click link in the left panel and it opens in the right one. Or you can open Gmail, Calendar and browse them at the same time.
Yes, you can open two windows and place them manually, but the point is to have that as a built in function - one click and its already done.