I run the latest Ubuntu live-CDs inside a VirtualBox virtual machine to test them and run the graphical installer. Without this keyboard shortcut, using the installer would be nearly impossible because its window is larger than the 800x600 pixel resolution that VirtualBox uses.
Holding the Alt key allows you to click anywhere in the window and pan it around the screen. Moving the installer up reveals the buttons at the bottom.
Once you have Ubuntu installed in the virtual machine, install the VirtualBox guest additions so you can select a higher resolution.