NB: This was made in Aug 2013, since then wordpress has improved its widget add interface (you will see a Manage in Customizer link at the top of widgets.php)
In WordPress the Widgets UI is great, but sometimes the widget you want to add is way at the bottom of the screen and the widget area (drop target) is at the top.
Usually, you have to start a drag and go to the edge of the screen and wait for the browser to scroll down, increasing the pain and chance of error. And If you need to add 3 ads / text widgets, then its doubly painful.
Hence, the first admin tweak for the Widget which resizes the left & right divs to the available area and makes them scroll.